After you've put down your floor, let it dry completely. This will take a few weeks, depending on your climatic conditions. It's best to stay off it. Protect the floor from direct sun and from freezing. A layer of loose straw will help protect the floor. This will slow down the drying some, so you may want to remove the straw when the conditions don't warrant protection. If you live where it's hot and dry, it might be a good idea to cover the floor with tarps or straw during the day to slow the drying and minimize cracking.
If you have to walk on the floor while you're working on it or before it's completely dry, put down 2 foot squares of plywood and step on them or you'll have footprints.
This is a good time to get firewood, put the gutters up, work on the drains, finish the porches, plant the fruit trees, visit non-cobbing friends and family, or go sit down and read a book.
The sealant for earth floors needs to soak into the floor rather than forming a hard (crackable) skin on top of the floor.
The basic recipe is boiled linseed oil (if you have an aversion to linseed oil, you could try other oils, possibly coconut?) and solvent (turpentine, mineral spirits or a citrus solvent). The solvent thins the oil and helps it soak into the floor. This sealant looks good on stone and brick too. Experiment!
CAUTION!! Oils and solvents are very flammable and can spontaneously combust. Be careful!
The first coat can be pure oil, because at this point the floor will be very porous. Adding solvent helps it soak in after the previous coat has made the floor less porous. The second coat can be 3/4 oil to 1/4 solvent. The third coat can be 1/2 oil and 1/2 solvent. The fourth coat can be 1/4 oil and 3/4 solvent. (This last mixture can be used whenever you want to increase the shine of the floor, like maybe once or twice a year.)
Let the floor dry thoroughly before sealing.
Warm the mixture in a double boiler carefully. Stop before the mixture gets to the point of smoking. Paint it onto the floor while it's still hot. It will absorb better if the floor is warm too.
Put the coating on with a paint roller or a large brush. Use a brush around the edges just like you would if you were painting. Let each application dry, then put on the next one. Apply as much as the floor can absorb and no more. If it puddles on the surface, mop it up.
If you want a really polished look, try heating up beeswax mixed with a little solvent and oil. Paint this on as a final varnish for the floors. Experiment on your floor tests, or in out-of-the-way places on your floor until you come up with a finish you like.