Simple joints are all well and good but how about making more complex shapes? Well the basic principal is the same - cut your pieces out, either from sheets or pipes and hold them in place before soldering. However you can run in to problems when you start soldering several pieces together - when everything heats up all the joints can melt and you piece can fall apart!
This chap was particularly prone to that sort of thing.
The way to avoid everything falling apart when you try and solder pieces that are close together is to conduct the heat away from the joints that you want to retain. I've found a good way to do this is to attach crocodile clip to the joints so the heat transfers into the clip rather than the solder. Here's an example.
Another option is to wrap wet cloth around the joints so the water evaporates and carries the heat with it. That works OK for larger pieces but not so well for small scale stuff. There's also a possiblity of it catching fire too(!)
By clamping off various pieces you can build quite complex shapes.