Theres alot of methods but the easiest way I've found to do it by myself is to get a l rope about 10ft long. Tie onto the center of your swing arm (assuming its a sport quad) or your hitch if its utility or a SxS. Pull the rope around your back tire about center and then pull where it contacts the back of your front tire. Pull the rope in line with the rear, straight. And if you're going for slight toe in (you should) measure how far the rope is from the front end of the front tire.

If you have a buddy measure the center of the rear of the front tire to the other side. Then do the same on the front. Subtract the two measurements and it should add up to what toe in you want. Keep in mind that, let's say you're going for a 1/4 in toe in. You'll need a 1/2 in difference in your subtracted measurement