1. Always read your manual, and/or assembly instructions first, before you attempt to assemble. In fact, read it before you do anything else.
2. If tools are required (this information will usually be in the manual or instructions) make sure you have them to hand. There is nothing more frustrating than suddenly finding that you need a socket wrench when you don't have one, the shops are closed and your workshop floor is covered in bits of machinery. Spread the tools and anything else you need so each is clearly visible, safe and ready to use.
3. Make sure you have adequate space around you when you start to assemble. Allow for the expected size of the machine once assembled. It seems obvious, but is often overlooked but make sure your exit from the assembly location is wide enough for the assembled machine to fit through.
4. Don't test any power-driven part of your machinery indoors, whatever the temptation and NEVER switch your assembled equipment on indoors.
5. Never drink alcohol before or during the assembly of machinery, and make sure you are fully alert and fit. However, a cup of tea always goes down well.