Ever heard that saying "Measure twice, cut once"? Kinda of the same step.
Plan, plan and plan! Draw you plans out (as you have the measurements right?) on paper.
Then transfer your design to the garden. I personally use chopsticks and balls of wool as both are cheap.
Measure from your wall/fences etc and stake with a chopstick. Once you have 2 stakes connect the 2 stakes with the wool to make a visible line. Obviously a bright wool is great at this point (I use red or orange).
I drew my ideas in Sketchup.
As they say in the Army: "Proper Planning Prevents P&#% Poor Performance!"