Okay, here's the best way to do it, IME, that is. You will need to take out the water, there's no avoiding that. Use, if you have them, the large tank-only trash cans that most use to mix up new batches of RO water. Siphon or pump out the water from one of your tanks into the trash can(s). Then place the sand in. Depending on the depth of the DSB, you can leave in some water, but be sure it won't get the top layer of sand, you want a DRY top layer. Here comes the tricky part. Once the sand layer is even or sloped like you like it, you need to place a thin, large plastic covering over the entire sandbed. When I did this in an old 20G awhile back, I used saran wrap because that's all I could get ahold of in a pinch, but you can probably find something non-toxic at a Home Depot, Lowes, or Menards. However, if you don't find anything, just go get a thing of Saran Wrap and indeed use that, as it works well. Once you have the sandbed entirely covered, and don't skimp on the covering, place a lot of small rocks, or anything heavy enough that it won't float once it gets covered in water, and place them on the covering near the corners, edges, and any areas that you needed to overlap the Saran Wrap or alternative. Them simply fill the tank. Once it is full, remove the weights and then take the plastic off. Be sure to simply cover the sandbed, and not to tuck it into corners or edges, as if you do this, it will disturb the sandbed and may cause minor clouding issues. Once you have the first tank done, use the trash can for the other tank, and repeat the process.
