Proofs must begin with an overview if they are long.
Bottom-up presentation
Proof of theorem 3 uses proof of theorem 2.
Proof of theorem 2 uses theorem 3.
Conventional order.
Top-down presentation:
Proof of Theorem 1 uses Theorem 2.
Proof of theorem 2 uses Theorem 3.
``Forward reference''. Provides motivation for reading theorem 2 etc. Acceptable.