This is my way, take from it what you will, the best advice I can give is to pick and choose what works for you from any advice.
Supplies: 2 cheap new/used power heads, 2 5g buckets, 1 small 25 or 50 watt heater, a sharpie permanent marker, 1 measuring cup (I prefer a 2 cup size) cheap plastic dollar store works just make sure your significant other knows it is strictly for mixxing salt, 1 small 1/4 cup measuring cup you will use 1 time (safe to use one from the kitchen just don't get caught or you may hear about it!).
Step 1: Mark the outside of your buckets to a full line, but make sure they are at the same height. Anywhere works but keep in mind salt, heater, and powerheads use space so a little low is better.
Step 2: fill the buckets with RO/DI to the line, submerge powerheads and plug in.
Step 3: pick a bucket to heat and put in the heater bringing it to tank temp.
Step 4: add a leveled 2 cups of salt and take a reading, you will be low.
Step 5: useing the smaller measuring cup add small level amounts until desired salinity is reached, Normally will take 2 or 3. adding it up make a mark on your 2 cup measuring cup.
Step 6: repeat 1-4 then useing your mark finish adding salt and check salinity (this is just a double check really), if it is right you can now mix salt very fast from then on.
The heater is used the first time this way so you get the salt mix correct after this time you can just drop it in before the change. I leave the powerheads in and on for 24 hours then pull them. I let the mix sit for a week and start the powerheads again the day of the water change adding the heater close to change time, after the change I mix a new batch and don't worry about it until next week.
In your case feel free to use 2 brands during your salt mix change and you can control ratio's until your old brand has been replaced.