sit down on a chair, place the bottom of your palm against your hip bone, point your index finger down the center of your thigh, spread your middle finger out, insert the needle (1 inch, 23 gauge would be good), pull back on the plunger to see if you've hit a vein, if so pull out a bit, or start over, otherwise inject.
generally you use a 1.5 inch in your butt, injecting up to 3 cc; 1 inch in your thigh, injecting up to 2 cc; and 1/2 inch in your delt, injecting up to 1 cc.