Upranal,
There are more experienced people than me that may be able to point you to a relevant website but I will try and give you a basic explanation.
I will not try and explain the "stand off" eliptical technique for 1/2"up to 2"and sometimes 3" as this is a little bit more involved.
I will use a 4" pipe butt weld as an example.
Code requirements will determine the number of shots required.
If we look at the most common scenario we will have 3 shots per butt.
Mark 0 at 12 oçlock on the top of the pipe.
Mark 1 at approx 4 oçlock.
Mark 2 at approx 8 oçlock.
First shot will have the film equally overlapping the area from 0 - 1 with source placed at 2.
Second shot will have the film equally overlapping 1 - 2 with source placed at 0.
Third shot will have the film equally overlapping 2 - 0 with source placed at 1.
The easiest way to work out placement of film and source is to take the circumference of the pipe and divide by the number of shots required.
Hope that helps,
Regards,
Shane

The ASME BPVC Section 5 has some good sketches of film placement and source position for pipe welds.

FYI-Kodak makes a nice RT training handbook available free on-line in .pdf format: "Radiography in Modern Industry"...(an oldie but a goodie) - although not much on pipe welds, has good info.