Step 3: Motor/battery assembly
Pic 1: if your motor has a plug on the end, like this, cut it off and strip the wires. Many motors don't have wires attached, but just a pair of terminals (usually a little metal tab with a hole in it). If you have this kind, solder a little bit of wire to one of the terminals that you'll attach to the battery holder - but I'm getting ahead, that's the next step.
Pic 2: protective plastic cover. You just need something to cover the upper part of the motor shaft as the battery holder protects the lower part. I used half of the bottom of one of those round cases you get out of gumball machines. They have a cheapo ring or keychain or doll or such inside. I never have any use for the contents, but the containers I use constantly for all kinds of things. They come in 3 or 4 different sizes and cost like 50 cents. The larger sizes you can also get in drugstores, where they contain pantyhose.
Pic 3: what it will look like, pre-glue
Pic 4: hot glue the motor on
Pic 5, 6, & 7: hot glue the plastic cover on. you can be terribly sloppy about this, it does not need to be air-tight or anything. only make sure that the motor can turn under the cover, and there are no large holes near the shaft.
It's a good idea to test at this point. Put batteries in the holder and hold the two motor terminals against the two battery terminals (this may require extra wire and alligator clips depending on the configuration of your components). If the motor doesn't go around, try re-setting the covering. Other possible problems are dead battery, dead motor, or bad connections. This last is highly likely because you're only holding them together at this point, but you should be able to get a little bit of motion even if it's intermittent.
Works? Good. Take the batteries out.