Here is a quotation from Leisureguy's Guide to Gourmet Shaving
Rub the stick vigorously against the grain all over your wet beard, including your upper lip. This will put plenty of soap on your face—and, of course, the thickest and toughest part of your beard will scrape off the most soap: a plus. Rubbing your beard vigorously with the wet brush will start to build the lather. Add a little hot water to the corner or center of your brush and continue to work the brush on your beard until all the soap that was applied has been worked into the lather and you have the lather you want. Again, experiment to find the amount of water that works best.
Special thanks to Michael Hamm for the quote from his book and this tutorial.
Step 1: Beard washed and wet
Step 2: Shave stick has been rubbed against the grain all over the beard
Step 3: Brush has just begun to build lather on the beard
Step 4: Brush has completed building the lather
Lathered brush holds plenty of lather for later passes