Hi, I am flying Hong Kong - Pekin (t2) and Pekin (t3)- Estambul, which is the procedure?

It's international to international transfer from T2 to T3. You don't need to have visa if your stopover is within 144 hours.
And the correct transfer procedures are the following: Flight Arrival - Transfer Procedures - Border Inspection - Security Check - Custom Inspection - Flight Waiting and Boarding