I'm no expert at truing wheels, but I can usually do it. Here's what I've learnt, mostly the hard way:
If it's a steel rim, forget it.
If they're not stainless steel spokes, the nippes will probably have corroded onto them; forget it.
Remove the tyre and tube before you begin.
You may be able to use the brake blocks as a guide to truth. If they won't stay put, use a clothes peg clipped onto the frame.
Use a quality spoke key that won't round the nipples off. Don't use the kind that has a dozen different size notches, becuase you'll use the wrong notch and round the nipples off. And if you don't, you'll spend more time searching for the right notch than actually working. This is bad for your sense of calm.
It's vital to preserve that sense of calm. As soon as you feel the slightest bit impatient, leave it till another day.
Remember that to tighten a nipple, you turn it clockwise AS SEEN LOOKING AT IF FROM OUTSIDE THE RIM. Imagine tightenng it with a screwdriver from outside the rim.
Don't actually do this, though. In fact, leave the rim tape on so that, if a spoke should snap, it won't shoot out and spear you through the eye, because that hurts.
Half a turn at a time is plenty.
When you have the wheel true, see if it's still circular.
Read
Sheldon on the subject. (But you already did this, right?)