Brackish water is simply partly salty water. It isn't as salty as sea water, but has more salt than freshwater.
The easiest way to make brackish water is to use half sea water and half freshwater and mix them together.
Bumblebee gobies don't need much salt. As long as the pH is above 7.0 and the GH is above 250ppm, they are pretty good. You can then add 2-4 heaped tablespoons of rock salt, sea salt or swimming pool salt for every 20 litres of tank water, and they will be happy.
Bumblebee gobies don't eat much dry food and prefer live or frozen (but defrosted) foods that sink to the bottom. They are small peaceful fish but sometimes nip fish with long fins. Good tank mates include small peaceful surface dwelling fishes that like hard water with a pH above 7.0 and maybe a bit of salt. Otherwise keep them in a single species tank.