For the only current 3.5 sheet in the repository, you would fill in the blanks for any character class you would use, regardless of whether it is a custom class or one from the official rules. It is designed to adhere to the official rules, in particular the original Players Handbook, but a lot of customization can be done one way or another. You may have to add custom skills (or skills from other resources) to the repeating section at the bottom of the skills tab. If all characters in a campaign use those skills, create a "master sheet" first, add those skills, and then duplicate it to create the characters to save on typing in the same information multiple times. You may need to edit some of the macros to adjust for a particular character's skills/attacks/etc. All the roll macros available on the 3.5 sheet should have a field available to edit them in though some of them require a check box be toggled to reveal the field. If you need help with macros, there are many folks on the Roll20 forums who are happy to help. If you want to play with the sheet, you will need to create a 'campaign' to do it in, but you can create more than one campaign and anyone can create one. That is also a good way to learn how to use the table top. :) Hope that helps.