This is the way I do it, but there are multiple options:
For a true bias, cut a square 36"x36"
Fold in half on the diagonal. Starting with the point facing you, fold the pointed end up so it's even with the folded edge.
I usually start in the center if I need longer strips, but if you need shorter lengths, you can start at the ends.
Line your ruler up perpendicular to the folded edge, and make your first cut.
Then start cutting whatever width measurement you need measuring from that cut edge.
And starch is ALWAYS a good thing.
I'm curious to see what you're making.