The iron cross is a very advanced calisthenics move. For this exercise you will need rings or TRX straps. Grab a set of rings and get into a position where your arms are fully extended and locked. Keeping them extended let the rings go out to your sides until they
Find a wall and kick into a handstand, letting your feet touch the wall. Use the wall to help you balance and lower your body up and down. This is an assisted handstand pushup and helps you build the strength to do normal ones.
Get into a handstand position and lower yourself down as far as you can go. Then push back up. Handstand push ups are a pretty advanced movement and should only be performed if you have the adequate strength.
Assume a normal push up position. Bring your feet closer to your body, similar to a handstand position but with your feet still on the ground. Lower down and up, again similar to a handstand push up.