Even if you're not going to need much, just plan to make a gallon. It's cheap.
Start out with 1/2 gallon of flavorful liquid. I went with 3 pints of apple juice and 1 pint of maple syrup, but you could as easily use a half-gallon of juice and a cup of dark brown sugar. Put in 1/2 cup salt. Season with a dozen or so whole peppercorns and whatever herbs you like (I used about 1 tbsp of an "herbs de provence" blend).
Bring to a simmer, ensure all salt and sugar is fully dissolved and take off heat. Let cool and add 1/2 gallon (4 lbs, actually) of ice to the warm brine.
How much brine you need depends more on your container than how much meat you're doing. There's a lot of wing-it involved here, and a lot of flexibility. Put the meat in your container, and pour in enough brine to cover it. Ziploc bags make for very efficient vessels. Let it soak. The rough advice for chicken is 1 hour/lb, for pork up to 4 hours/lb. Shrimp does well in a brine too, say 20 minutes to a half hour.
Save any un-used brine if you wish.
Pull the meat out of the brine. Quickly pat it dry with some paper towels and get it into/onto your cooking vessel and cooking. If you don't, its own weight will start forcing out the brine.
Dump used brine; it's not good for anything I know of.
The rest is down to what you're doing. For roast pork loin, set a meat thermometer to alert at 160 degrees. That's around medium-done, which is good for pork loin. Let it roast. Pull it out when the thermometer beeps and rest it for at least 10 minutes, 15 would be better.
Oh, and remember...