Tie the two sticks into a cross with yarn or string.
Make sure there is a long piece leftover so you can hang it up.
Then hang up the cross from a branch or shelf.
Attach yarn or string to each natural item you want in the mobile. Then tie them to a stick.
As you add items, you'll need to move them out or in on the stick to get the mobile to balance. Keep playing around with your items until the whole mobile is even.