Depending on the pin, and assuming all parts are there:
1. Unplug the machine, and feed the power cord back into the body of the cabinet (depending on the fitting)
2. Open the coindoor, unlatch the lockdown bar, and remove the playfield glass. The playfield glass should be tempered or safety glass, so be careful when setting it aside on it's edge so it doesn't shatter. Remove the pinballs, coinbox, and anything else that is loose inside the cabinet. Leaving the pinballs loose in a machine when transporting it could break plastics as they roll and fly around. Replace the playfield glass, lockdown bar, lock the coin door, and put all the keys, pinballs, etc. in the coin box for transport.
3. Make sure the backbox latch is set (at the rear of the machine) so the backbox doesn't fall down accidentally.
Lift out the backglass (the key for the backglass is often hanging on the inside of the coin door), and set it aside so it doesn't get broken. Lift out the speaker panel. Undo the head bolts (inside the bottom of the backbox), and put them in the coinbox. Replace the speaker panel and backglass, locking it, and put the keys either back on the hook on the inside of the coin door, or in the coinbox for transport with everything else. If you care about your floors, take the time to remove the backbox latch and put it and the hex screws in the coinbox.
4. Place a sheet of cardboard or foam on the playfield glass, and carefully tilt and lower the backbox down on to the main cabinet. I like to strap the head down, and put cardboard under the strap at the corners so the strap doesn't rub the paint. If you use a strap, don't reef it too tight - just snug enough that it won't move. Now is a good time shrinkwrap the whole machine for protection against damage while moving the pin through narrow doors and stairwells. If you don't use a strap to secure the backbox, a good 5 or 6 passes of shrinkwrap should secure the backbox.
5. Remove the rear legs first, and lower the rear to the ground. Tilt the whole machine up on to its back, and remove the front legs. Put the leg bolts in the coinbox. If you use a two wheel dolly, leave the pin on it's back and strap the dolly to the pin.
6. Once it's at your vehicle, I like to load them with the coinbox towards the front of the front of the vehicle. Use cardboard or a moving blanket to protect your bumper and interior from scratches. Pins are heavy. While it's possible to load them on your own, I don't think it's worth the risk of damage to myself or the pin, and I get a buddy to help out. Ensure that nothing in the back of your vehicle could slide around and damage the pin while in transit (eg. your dolly, the legs, etc.).
7. Check to make sure you have all the legs, bolts, manual, and other parts before you leave the seller's house.
At your house, you just do it all in reverse. If you didn't use a strap, remember that when you cut the shrinkwrap off, the backbox could fall if the pin is on it's back. Give the game a good cleaning and wax the playfield. Get your level and inclinometer out on the playfield (glass off), and adjust the leg levellers until the game's dialed-in and ready to play.
There are many far more experienced pin movers than myself, so if I've left anything out, or there are other tips, I'd love to hear them too!