I've soldered a few small jobs in the past but now am having problems.

I feel I am missing something basic that did not come into play on my previous work.
I'm assembling 3/4 and 1/2 copper supply pipes for a new bathroom.

Maybe someone can clarify and/or answer some of these questions:

1) I suspect the problem has something to do with even heating of the pipe.
- Is there an optimal distance to hold the torch (propane) off the pipe and joint?
- I am heating the pipe first and then the joint. Is it the reverse? Does it matter?
- There seems to be a small window of time that the pipe is hot enough to melt the solder but I still get solder running around a horizontal joint and dripping out the bottom without being carried into the joint (I'm only using a thin coat of flux).

2) I am trying to assemble the major sections of pipe on a sheet of plywood. This helps keep a long assembly of pipe with angles square.
- Should I be putting something under the pipe to separate it from the wood?

3) If I have several joints in close proximity should I place and solder each one separately or assemble the group and move from one to the next?

4) My understanding is that the soldering process works by heating the pipe so it melts the solder and once liquified it is draw by capillary action into the joint.
- Is there something else going on?

Yes ... I'm feeling like a beginner.

Thank you.