If you refer to ir2153 smps, primary will be 12 + 12 which means total is 24 laps. core = EI33/35.
1. CT must exist : yes, Look at sample trafo at old psu, you will know the answer how they hide it.
you cant do with single winding 24 round : NO. You need to provide for example, 12 + 12, coil A and B wind together until u get 12 laps total, which means coil A have 12 laps and cable B also 12 laps. which means 12 + 12 = 24. Remember at ending of each coil that we call A1 and B1, so read previous post how to connect them.
2. V to P is 4 turn, its aux supply for ir2153 when its started, but without it smps also will work.
I know it would be little hard work and understand for first time build smps, i`ve face it few years ago and at that time no one give me correct answer even they just ask me to try and error which sometimes blown my smps hehehehe, nvm
. But if you successful understand how to wind trafo correctly then you`ll happy and more easy to build smps. So here in Diysmps have some tools to calculate primary and secondary turn by each core type and size here :
http://www.diysmps.com/forums/showthread.php?275-SMPS-transformer-design-tool
regards