It's my first time grouting a floor (5x9' bathroom). The tiles are 2" hex.
I figured I would only be able to do half the floor at a time to avoid disturbing the grout when it's soft. Is this true, or is there a good way to do the whole thing at once?
You need to start at the furthest corner from your exit path and work your way out. Grout the entire floor at the same time or you may see color differences where you stopped then started again. Mix all the grout as a single batch (that is a fairly small area, it should not be too difficult).
Use your grout float to remove as much excess as possible as you work. I keep several buckets of water and several sponges and do a quick damp "dirty" sponge to the grouted areas as I work. You can use a spray bottle to give a slight misting to completed areas if you are concerned that you cannot work quickly enough. Cover unused grout in the bucket with plastic wrap, you can use this to repair any grout lines accidentally disturbed during the next steps.
When complete, let it set for 15/30 minutes then proceed with your sponging. It is important that you keep a "dirty" sponge and bucket (initial wipe), and a "clean" sponge and bucket ("clean" wipe). Wring out and rinse sponges after each wipe, and change water frequently.
Let it set overnight, then do a "haze sponging". Mix a small amount of vinegar (or muriatic acid) with clean water in a bucket and sponge the entire floor to remove remaining grout "haze".
Simply work on a scrap of plywood, which will keep you above grout level. Grout is nearly always slightly recessed.
Most installers simply keep their shoes flat on the tile, which accomplishes the same.
Mix all your grout together while its dry and mix thoroughly. This way all the grout will be the same color, then you can add your water and the correct amount of grout needed. Simply grout the entire floor and allow to dry 20 minutes and then start your sponge and wiping processes. I would highly suggest applying a solvent clear sealer over the grout.