i assume you dont have your tank yet....
("i used to have a 30g...
would like to try again in the near future...")
...so you have 2 choices...
set up your tank with salt in. like pufferpunk said a SG of around 1.005 is best for figure 8 puffers. leave it to cycle for a good few weeks.
then go and buy your fish. aclimatise them in a bucket using the drip method. when the SG is the same in the bucket as it is in the tank, then add your fish.
i left mine for 6 weeks when i first added salt. and then raised it from 1.005 to 1.010 over a few weeks. but you obviously wouldnt need too raise it further.
this way you will give your benificial bacteria a chance to colonise.
alternatively, you could set up your tank as freshwater, and raise the fish in the tank slowly up to 1.005, over a few weeks. but you will still need to leave your tank to cycle anyway. so dont think thas way will be quicker
dont raise the salt too quick though, or most of the bacteria in the tank will die, and so will your fish.