If you don't have access to a either a proper truing stand or even the materials and/or skills to assemble a makeshift stand like others have suggested, you can simply use a sharpie to mark the spots to adjust.

With the bike upside down, I begin by unmounting the wheel and the tire from the wheel then remount to the frame. Whether it's a rear wheel or front wheel, mount it in the rear because that's most stable. Then I hold the sharpie closely on the seat stay, either side, pointing towards the braking surface and spin the wheel. Make my adjustments at the marks, wipe off marks with an alcohol pad, grab sharpie and spin again. Repeat as necessary.

I usually do it a couple times on each side until it's fairly balanced. This isn't as good as using a proper stand but it'll get you close. I can get mine near perfect.

And if you're having trouble remembering which direction to turn the spokes, try keeping the spoke you're adjusting facing you. That way you can see how righty-tighty, lefty-lucy applies. Or by keeping the nipple pointing downwards, at the top of the wheel, it's the same concept designed to keep focus. I always screw it up with the nipples pointing upwards, bottom of the wheel. It's kind of like backing a trailer. I can't do that either.