You do this with your eyes closed...

Just make sure you got nice, clean surfaces before mounting gaskets. and spread out the pressure on the valve cover when you tighten the bolts.
Use some solvent on a piece of cotton to clean up surfaces. A putty knife can be a help to remove old gaskets.
Old covers like valvecovers, oilpans etc. you can check that steel around boltholes are straight and in line with the rest of the covers edge, that these bolt areas not are below rest of the cover edges(down to heads) or else it may be that you dont get cover tight enough to seal... if so, just put the covers upside down, use a hammer and knock a little easy with something to straighten out the areas around the holes a bit... (maybe a bit upwards (from the heads)
(this is rare do, but its easy to check. just hard to explain it in english)
