My wife and I love art and have a nice collection, so where to place new art is important, and hanging it so it won't crash to the ground is also important. At my house I am the picture hanger and this one of those areas that can be intimidating even to the experienced crafty handy person who is capable of home repairs and making stuff. Here are a few tips to make this go smoothly.
Have a nice Level, this one is about 9 inches and I think I had it for about 12years very simple. Generally I use various nails to hang pictures depending on the weight. Even if your walls are sheetrock you can still hang an 8X10 or 11X17 frame with a couple of nails placed at an angle. For pieces that are bigger heavier I love this company: Ook Picture Hangers all sorts of hardware for hanging. Most hardware stores cary the Ook variety picture hanging kit.
Choose the spot to hang the picture, just eyeball where it should be placed. Then behind the frame I will put painters tape on the wall one horizontal piece with the vertical coming down from the approximate center. Take a pencil and draw on line right along the top of the frame, then I'll make a little cross at the center will be. I've been doing this for a long time so I can usually get the center with out measuring, depending on how confident you feel you can measure to find the center.
Now on the back of the picture use a tape measure hook it on to the wire, or the bracket find the distance from the wire under hanging tension to the top edge of the frame. This was about 2 1/4"
Back to the tape find where that 2 1/4" mark will be. Make a hash mark with the pencil.
Now use the level to create a nice level line about 2 inches for each of the two nails.
Remove the tape and hammer the nails in at about a 45 degree angle. The tape keeps it all clean so if you remove the picture you only have the little holes to patch up without a bunch of pencil marks.
Finally done! Now I can get on with my Saturday. Use the level to check the if the picture is straight, this one is not perfectly level but I matched it to the other frames. My rule of thumb:
"If you have to stare at it for more then 30 seconds to tell if it's hanging crooked, it's fine." And as it is with everything the more you do it you will become an expert. Then unfortunately everyone will start enlisting you to hang their pictures.