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 will usually be in the manual or instructions) make sure you have them to hand. Thee 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.
3. Make sure you have adequate space around you when you start to assemble.
4. Don't test any part of your machinery indoors, whatever the temptation and ON NO ACCOUNT ever switch your assembled equipment on inside.
5. Never drink alcohol while you are assembling machinery. However, a cup of tea always goes down well.