Introduction
This tutorial page was originally written a couple of years ago when WAX was my system of choice.  In the time since, WAX has effectively fallen off the map and been replaced by other, larger chains.  Things like SOL and Polygon.  I've been working with Polygon on OpenSea for a few years at this point.  It is the system I like the best, and trust the most.  As such, I determined that it was time to rewrite this tutorial from scratch and show how things are done now.  Well, how I DO things.

The WAX version of the tutorial is still here, if you are interested in reading that.

Update 2024-09-23:  I have started the process.  Screengrabs are in place and in order.  Now I am updating the text.  At this point, most of the text looks good.  Just checking for spelling and grammar.

What are we going to do?  
As I mentioned before, there are a lot of blockchains that support NFTs. There are a lot of marketplaces. There are a lot of ways to do this.  For this tutorial, I am going to stick strictly to creating, minting, and selling an NFT on the Polygon chain using OpenSea for minting, and as a marketplace.  That's the super-high level explanation.

When you are done, you will have a page like this one, with YOUR NFT in YOUR collection.

I will explain each part, step by step.  And you'll need that, as it's a lot of steps, and there isn't much explanation from those systems.

Do yourself a favor and use your laptop for this.  The page is mobile friendly, but really, it's WAY too much to fit on a phone.  Besides...  You can't really follow along that way.

Getting started - First thing's first - The things you will need
Install MetaMask.  Create new wallet.  Make sure you have 1 POL (formally MATIC) in your wallet.  You will have to pay some gas fees during the process of this tutorial.
So you understand how this works...  MetaMask is your wallet.  Your wallet is your proof of ownership for everything you do on the Polygon network.  All transactions use this wallet, and are signed (authorized) using it.  Your wallet also holds all your coins (tokens), and can be used across muitiple chains.  In this case, it will be used for Polygon.

MetaMask installs as an extension on your browser. Any sites you go to that support connections will prompt you to connect your wallet.  This of course, includes OpenSea.

Create your account on OpenSea.  Connect your MetaMask wallet. 
Rollover the little round icon at the top right of the screen, and this menu will popup.

Go to settings.  That will take you to this screen.

Fill out this form.  You are going to see places throughout this tutorial where you will need to put some sort of image for the sake of displaying this stuff.  Just follow the specs provided on the page.  Once you fill all of this out, you will be asked to validate your email.  That is how you activate your OpenSea account.

Now that your accounts are set up and ready to go, we can take a break from that for a moment to work on creating your first NFT.

Create your image
Decide what format you are going to use.  They all must be the same.  If the first one you mint is a PNG, then all subsequent assets must be PNGs.  NFTs can be anything from music to video to images of all kinds.  For the purposes of this tutorial, we will stick to creating a PNG image, and a collection that will use PNG images.

How do you create your image?  That is really up to you.  But if you lack the ability to take a picture or draw something yourself, generative AI is a good place to start.  I just happen to have a page with such an AI image generator on it.  It's not the best or the fastest.  But it's 100% open source and royalty free...  And yes, that is how I created the NFT for this tutorial.

Now that you have created your image and saved it as a PNG, you are ready to start working on your collection.

Rollover that little icon at the top right again, go to Studio. 

Since you have no collection for which to mint an NFT yet, you will need to create one.

So, you will see a page with this

Click the Create button.

This is an intermediate page asking what sort of minting you would like to do.  Click on "Collection or Item".

This will bring you to this page.  This is the Mint new NFT page.  But you still have no collection.     I really don't know why OpenSea set up their order of operations like this.  It is what it is.  And you only need to do this once.  The next time you come to this page, there will be a new collection in that dropdown box.

Choose "Create new collection".

Finally.  The Create Collection page.  From here you will do your basic setup for your collection.  We will revisit this later, because there is more to do.  But for now, this will create your contract for your collection.  Feel free to click on the little help links on these pages like, "what is a contract?", if you would like more details on what we are doing here.

The final step at the bottom is to choose what chain you want to work with.  Ethereum and Base are shown by default.  Click the little "more" icon to show others, and select Polygon.

Once you have that page filled out, and you are satisfied with the basic setup, click Save.  This will call MetaMask to approve the creation of the contract and tell you how much gas you have to pay.

Pay your 0.01 Polygon and Confirm.

Now that this is all done, when you go to the "Created" tab in OpenSea, you will now see that you have a collection into which you can start minting assets!

We are finally ready to create our first NFT.  Follow the steps above to go back to Studio and Create an NFT.  The dropdown will now show your new collection.  Choose it.

Drag and drop your new image.  Give it a name.  Specify how many you want to mint.  You can mint anyhwhere from 1 to billions.  But the more you mint, the less they will be worth.  I'm going with 10k for this one. Give it a description.  Give it some Traits. 

The Traits are a fun little thing.  You can create any number of them.  You can name them whatever you want.  You can assign whatever value you want.

The idea is that if you have a big collection, some of your assets will share some traits.  Like, "Type / Animal".  With that in place, people viewing your collection will be able to filter down to assets that are of that type - Animals. They share that trait. 
That is a wide open sandbox for you to work with.  The link I provide there shows an example of how one of my collections is set up.

One other thing to note here.  Once you mint this NFT, it cannot be changed.  Nothing can change.  All the data you enter here is locked.  So be absolutely sure that all your info is correct before you finalize it.

Finally, mint the thing.

Watch the gas fee on this page.  Polygon fluctuates.  The fee for this is usually about 0.005 to 0.008 POL. It shouldn't ever be much higher than 0.009.  If it is, the network is busy and that makes the gas fees go up.  I've seen the gas fee go as high as 0.5 POL!  And that is clearly ridiculous.  When that happens, I just walk away and wait until gas comes down again.

Anyway... Be careful to look at that number.

You did it!  You have your first collection and your first NFT minted!  Congratulations.  Welcome to the club.  The next thing you want to do now is put the thing up for sale!

Click into the "View Item."  That will bring you to the main page for your NFT.

List 1 Item

Decide how many you want to sell, Set the price (make sure to select POL).  Set the length of time to sell it.  6 Months is normal. Click on Create Listing.

This will invoke the MetaMask extension to ask you to Sign (authorize) putting this item up for sale.  There are NO gas fees for this!

Once you Complete the Listing, the NFT can be bought by people visiting the page.

Collections Page
At this point, you are mostly finished.  All that is left to do now is some last bits of tidying up.

I mentioned that we would get back to spiffying up Collection Page.  Here we are.  The reason that this is set so far after the collection was created is because, as per OpenSea's workflow, this is pretty much where it belongs.  You can't really get to this until you create the collection, and mint at least one NFT.  So now that that's done, we can revisit the collection and add some new details.

Rollover that icon at the top/right, and go to Studio.  We did this before.  This page will look different now.

You want to click on Edit details.

Here you will be able to edit the details for this collection.  You can set the name, give it a nice detailed description, customize the URL, and give it some category tags to make it easier for users to find.

Once you're done there, save your work, and click into the Page Editor.  This is where you can set your icon and your banner.

That's about it.  I think I've covered this about as well as I can.  Feel free to hit the contact button if you have any questions.

I hope you found this helpful!  I hope you mint piles of NFTs and become a millionaire!

Please share on social media.  Join my Discord.  Have a great day!