Brackish Startup

While strolling through my LFS today my girlfriend caught sight of a green spotted puffer and fell in love. I have a spare 20G running with some guppies in it that I can convert but I'm not sure on the particulars of the process. I know I'll need a hydrometer, (unless I can calculate salinity based on salt added per gallon) and some marine salt. After that I'm clueless. Any help would be great.

I just have some common gravel in there at the moment. Thought I could avoid crushed coral and live rock with a brackish. Is it just like cycling a new tank when you convert or will any bacteria survive? Trying to figure out how long it will take to have the tank ready.