Prime your nails with a base coat.
Paint your nails with two coats of your base color.
Stick two pieces of scotch tape to your arm (to dull the adhesive) then tape off your nail so a triangle of polish is showing.
Paint your nail with your accent color and remove the tape immediately. This will give your shape crisp edges. If you wait for it to dry you may pull up the polish.
You can stop at one accent color or you can repeat the taping step, covering part of your accent color and paint with another coordinating color or even your primary color again.
Finish with a quick dry top coat and you're done!