At the beginning, we only used the signs I thought she'd think were important: milk, more, all done, and bath.
Eleanor started signing back at us between 10-11 months and it has been pretty incredible how much easier communication became with her at that point and how much less frustrated she seemed. We've since taught her more signs (sleep, thank you, please, phone, ball). I use other signs with her that she does not sign back but understands (pain/hurt, book, change, diaper, cracker, light, mom, dad). Daycare taught her some signs that we didn't catch on were signs until I realized how frustrated she was getting. She kept repeating the same motions at me (clean up, my turn) - these signs I had to google the action to see if they were, in fact, signs, and what they meant. She has also made up some of her own signs (dance, pacifier, the end). With the ones she makes up, they generally reflect the concept she's trying to convey. For example, her sign for "the end" is very similar to "all done" but with a lot less waving. Her sign for "pacifier" is the sign for "milk" and then tapping her mouth afterward. There are also signs that we sign to her (help, bless you) that she knows, but has made up her own sign that somewhat mirrors the actual sign, but the sign itself seems too complicated for her. All together, Eleanor probably signs about 20 different things, and understands about 30. It is certainly an imperfect process, but has been very helpful overall in her being able to get her point across.
So here is a video of Eleanor using her made up sign for "dance" which really means "play some music" (open palms facing forward, waving arms). She signs this any time there is a screen out (phone, ipad, computer) because she knows they play music. You can see how she gets increasingly emphatic with her signs when she feels they go unnoticed early.