Simple grabs are a mainstay of freestyle riding. You can spin and flip all you want but a solid grab with good amplitude and a stomped landing goes a long way.
Step 1: Find or build a decent sized jump. A good-sized jump for a solid grab is one where your time in the air is at least 2 seconds. Become familiar with this jump and be able to do a nice floaty straight air with a solid landing first. As always make sure to be wearing the proper protective gear like a helmet and pads and also make sure your landing area has a clear and safe run out. If there is a question on the run-out are always make sure you have a friend who is spotting that area.
Step 2: As you begin to jump and are leaving the lip (the end of the jump) concentrate on being centered on your board at first. Once you are nearing the apex (highest point) of the jump reach forward and grab the front truck. Do this several times. Get the grab, stay centered, let go and land.
Step 3: Now start to tweak the grab and put some style behind it. What I like to do is as I am leaving the lip of the jump I keep my board in that "pointing upward" position. This way the front truck is nearer to my hand, I can hold the grab longer and it accentuates the look of this grab.
Step 4: When you master the first three steps then try to combine this grab with another grab or incorporate it into a 180 or 360 spin. I like to do a front truck grab into a back truck grab. Good luck and have fun.