Doug has really covered all the main points!
And don't forget to score the folds first with a blunt blade of something similar. I also fold small parts against a small engineer's set square and also on a small piece of plate glass - this helps keep everything nice and square.
There's a selection of what I use - the knife is my blunt scoring knife.
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The old adage "practice makes perfect" still holds true - use some scrap paper and practice folding small parts for 30 minutes. You will be impressed at how dexterous you become!