There are several parts to this process. 
Part 1. The inventory file. We'll fill this out basically the same as if we were creating a parent. There are a few more fields to include. 
Part 2. Images
Part 3. Error codes, most of the error codes are going to be the same as the errors you'll encounter using the variation wizard or creating parents. I'll add a few more advanced errors to this section but if you don't see a solution here, you can check the 'Dealing with failed uploads course.

If you haven't added the images in the template upload you're going to want to do that now.
You'll need your own product images, you can't download and reuse images from other websites, that's a good way to get in trouble.
You'll have to either take your own pictures and edit them or pay someone to get pictures for you.

Some of the errors are simple fixes like missing information, if you're creating listings you probably know how to read a processing report and take care of the easy ones. If not, check out my 'Dealing with failed uploads' course for help with those and case templates for when you need seller support to pitch in.

This is for the more advanced problems and how to deal with them.