Best way is to use a sprinkler attached to the hose and spray gently. You can easily control water flow this way. Get the one that has the trigger. This will prevent the substrate from being stirred up. Some people also use a flat dish, and dump water on there to minimize the splash. Again, to deter it from disrupting the substrate.
I ended up using the ole plate and bowl trick. Worked great! Took forever to fill the 55gallon up but was worht it. Celo wrap= brand name Celophane. Didn't try it though. The sprinkler idea sounds great but I do not have a hose to use.