Use the tools you have! Your hands, your trowel, or whatever is handy. I know that my hand from thumb to index is about 6". If I want 3" spacing, I mark out the 6" first then put an additional mark between each. The "mark" is a hole I press into the soil where the next plant or seed needs to be placed.
If measuring in feet, I can use my handy trowel. It is 12" in length. If I want 18" spacing, I know it is one and a half trowel lengths.
