I want to make a mass giveaway for 25 steam keys, what must I do?

afaik for GA to be featured it must be at least 50 keys. Anyway you must write a Support ticket about it and they will direct you further :>

Mods probably. If they don't notice this and talk to you already you can probably find some in the SG group chat. ;)

If you need more giveaway slots so you can make a 25-copy giveaway, then contact support.

For it to be featured, you'll need 50 copies min though.

I can tell you from experience that 50 (or more) is the magic number. You will not be featured for less. You have to send in a support ticket if you want more slots, or just do multiple giveaways until they are all gone (probably the easier way).