Here is how you make a very simple and basic icon base.

The materials you need are a picture. Any picture! I'm using this one:

First take your selection tool and set it to square.

Once you've done, select the part of the picture you want to be your icon like so:

Now copy it and paste as a new image (Ctrl+V). Now time to resize it.

Set it to the size you wish.

What you have now is what people call a base.

Now you don't want to leave it as just a base. You can add borders and text and spiff it up. Get creative!