Visit your profile page on your Mastodon instance. Click Edit profile, either from the link shown under your Mastodon username, or by clicking the three dots to the right near your username and choosing Edit profile from the menu.

From the list of options on the left, choose Development. Then click New Application.

Give your application a name. It can be whatever you want it to be. Have fun with it!

Choose the scope of your application. It is good practice to limit the scope to only the specific things you need. The first section contains the read scopes. You only need read:statuses. Check the box next to it.

Scroll to the next section.

The second section contains the write scopes. You only need write:statuses. Check the box next to it.

Scroll to the bottom of the page.

Those two are the only scopes you need to allow. You're ready to submit your application. Click SAVE CHANGES.

You'll see a green rectangle letting you know your application was successfully created. Below it, you'll find your new application listed with name and scopes. Click on your application name.

There you'll find Your access token followed by the token itself.