"Freeze and fuse" is a term devised to describe a technique to obtain complex edge shapes and some bas relief.
The basic method used is as follows, although there are a number of variations that can be successfully adopted.
Mix enough water with fine frit to make a damp slurry.
Then place about 3mm into your mould and tap on a hard surface. Tap quite vigorously to bring any air bubbles to the top and compact your powder.
Use a paper towel at this point and blot off any water that has risen to the surface.
Continue to layer, tap and blot until you're level with the top of the mould.
The more you tap and blot out any water (and every time you tap, more will rise to the top) the better your results will be.
When your paper towel won't absorb any more water, you're ready to put your mould into the freezer. One to three hours should be enough, but it must be frozen throughout.
Take the frozen glass from the mould. Letting it sit while you programme the kiln will allow it to come from the mould more easily. Place the glass form on the kiln shelf. Raise the temperature as fast as you like to 90C. Soak there for at least half an hour to remove any water in the piece. Then raise the temperature at about 200C/hour (depending on the thickness and size of the glass form) to fire polishing temperature (between 720C and 760C). Higher temperatures will flatten the form and change its shape.
Some experimentation is required to get the best combination of rate and temperature.
Experience will show you variations on this basic method.