Nothing completes a good looking set of arms then a killer pair of forearms and therefore how to build forearms is a question I get a lot when I am in the gym or speaking with clients.
In order to get big forearms, you need to push them to the limit. To build your forearms, you need to do exercises for them about 4 times a week. The reason for this is that they are a very resistant muscle because of the amount of use they get. Think about it. You use your forearms for a lot of things.
Things like writing, holding things, lifting things, driving etc. This constant use makes your forearms very resistant and therefore if you want to build your forearms, you need to put them through a workout that they would never expect and push them to a level that exceeds what they are used to.
Great exercises that you can do at home or in the gym in order to get big forearms are Dumbbell Back & Forths and Dumbbell Twisters. Dumbbell back and forths can be done by taking a dumbbell in each hand and rotate the dumbbells to the right and to the left using and flexing your forearms. Do 4 sets of 25 reps.