wiring new outdoor outlet off exisint outlet

Open up another recep and see how it's done.

I generally do whatever is easiest.

As long as the connections are good and tight it doesn't really matter if you pigatail off the wiring or attach to the recep terminals.

You will have to pigtail the ground wire though.

Also if you do not already have a gfci on the circuit for the outlet inside the garage (which you should consider) you will definitely need one for outside. You can spend 20-30 dollars on a breaker for the circuit in your breaker box or 10 dollars on a gfci for the outlet inside and follow the directions to connect the old recepticle on the outside to the new gfci recepticle on the inside. The first time you grab a wet cord outside and plug it in you will appreciate the extra work.