Here is the process I follow to prepare a surface for a finished studio illustration.
The materials I use are Liquitex Basics Acrylique Gesso, a craft foam brush (I usually get these in a multipack at Michaels), double-sided hardboard (I get a 4ft x 8ft sheet and have them cut it into 4ft x 2ft strips, then cut it to whatever size I need on my tablesaw at home), Black & Decker Mouse electric sander, and an old hair dryer (optional)
Lightly sand the surface to remove any oils and imperfections on the surface.
Step 2: Mix your gesso with a little bit of water in a separate container to give it a creamier consistancy that is more conducive to smooth application
Step 3: Thoroughly clean the surface of any debris with a damp cloth
Step 4: Using your foam brush apply the diluted gesso evenly with smooth parallel strokes
Step 5: Apply perpendicular strokes to those you initially used to gain a smoother grain (you may need to use more gesso at this stage if you applied it very thinly on step 4) Do not press too hard with your brush strokes, lightly brush across the surface to gain a smooth surface.
I used to use a hair dryer to speed the process, now I use the drying time as a chance to take a break and get something else done.
Step 7: repeat steps 4-6
Step 8: Lightly sand the surface to your desired smoothness, and then clean surface of gesso shavings
Step 9: repeats steps 3-8 till you achieve your desired coverage and texture. I generally do this 2 more times so that I have a very smooth, bright white surface. Occasionaly I will skip the final sanding if I am going for a brushier look to a painting.