There are many ways to train grip.
-Stand upright and hold two dumbbells at your sides.
-Hold a barbell in front of you (overhand grip).
-Hang from a Chin-up bar.
-Grap the head of a smaller dumbbell and hold it out in front of you.
-Invest in a set of Iron man grippers. If you have never trained with them I suggest you start with the "Trainer."
-Make a wrist roller (a strong bar or piece of wood with a hole in the middle so that you can tie a strong string through it. Tie a plate on the other end of the string.) Roll it up and down.
Whatever you do, don't overtrain grip, this can be done easily if you are not careful.
Train grip the way you would train any other muscle. Three times per week is plenty, do 3 or 4 sets of any of the movements above.