A secondary base is there for support. You have two options on how to start in your load:
1st- place your right hand under the arch of your flyers shoe and your left hand at the top of her foot creating a tight, solid, stable hold.
2nd - place your right hand under the arch of your flyers shoe and your left around your main base's outside wrist giving her support.
Whichever one you choose is based on comfort level for you, the main base, and the flyer so try both out a few times to get a feel for the right fit for you.
From there you drive up powerfully, using your legs, and locking out your arms at the top. The key is to be as close to your main base as possible creating a good platform for your flyer. I also teach my girls to secondary with their shoulder against the main base's chest so that they are facing out towards the crowd but constantly looking up at their flyer.
You're perfectly safe in that position as long as you remember to turn and catch for cradles or falls.