I find one of the easiest ways to build nice, consistent Celtic knots is to use graph paper. This gives you an even guide to follow as you plot out your knot. I usually use graph paper where every second dot is slightly bigger, going both up and down. This leaves you with one regular dot, and then an emphasized dot, then another regular dot, and so on. You can bold every second dot yourself on regular graph paper by just using a different colored pen or marker to touch them up.
I have created a PDF sheet that you can print out to do your practicing on, if you have trouble making your own Knotwork Grid from graph paper.