I could write a really long post about this, but I'll start with the very basics.
This guy's stuff is good. Yes, it's literally just making circles daily, so people would tend to dismiss it as too simple, but I have personally tried this out and gotten a big improvement in mobility and dexterity of the wrist and hands. This will not build a ton of strength, but it's a perfect daily warm-up. Then paired with the other video, you'll begin to build even more range and strength.
Wrist starts at 6:27 -
https://m.youtube.com/watch?list=PLFB45 ... ode=NORMAL
Fingers start at 2:30 -
https://m.youtube.com/watch?list=PLFB45 ... ode=NORMAL
These are phenomenally effective if you do them right.
https://m.youtube.com/watch?v=-hlWgH3_0NU
I do them both arms at the same time. Make sure on the straight arm ones to keep your elbows locked. I also dislike his range on the extension ones, maybe because he's tight, but you should really fight for the last few milimeters. The objective is a minimum of a 90 degree forearm/hand angle the whole time during the extension part, and the fingers should eventually reach 180 degrees (straight) with the hands. I either do 1 set of 10 in each position, or 1 set of 5 with a hard 5 second pause at end range on the last rep of each position. Daily, or near daily.
Here's some TLC from stretching.
https://m.youtube.com/watch?v=GmMfW2kCkeE
There are two more stretches that can turn in to pushups that I like. Dorsal (pushups), and wrist rock (pushups). Can't find a video of the stretches, but there are pushup videos. The stretch is just the top position, but from the knees with hips bent, no pushup.
https://m.youtube.com/watch?v=oSkF3CCExnY
The goal here is to internally rotate the shoulder as much as you can, so the pinkies are pointing straight back at your knees or as close to this as possible, then LOCK out your elbows, and lean back into the stretch, maybe sway gently side to side as well.
Wrist rock - can't find a video, but from your knees, make two fists and place the top of your fists on the ground (instead of the palm). Palm sides should face inward (neutral). Lock elbows and rock wrist forward (ulnar deviation), then bend elbows and rock backward (radial deviation).
Between all those mobility and stretch exercises, you should have a very basic prepararedness for grip strength and a way to prevent and rehab injuries.
For strength, I could recommend a few different types of wrist pushup progressions but there's a pay wall between the videos. Gymnastic Bodies Handstand One program has a good progression. This guy has some different but also effective stuff that is way cheaper.
https://vimeo.com/awarerelaxedconnected/vod_pages
I've seen tons of different wrist pushup variations, and usually they want money, but if you can catch a quick clip of what it looks like, and understand progression, you basically have it all. For any given wrist pushup, easy: on wall, intermediate: high table, or from knees with hips bent to some degree, int. level 2: from lower surface, or knees with hips straight, advanced: from ground on toes just like a pushup (hips bent, then hips straight).
It's important to go REALLY slow and easy with wrist and finger strengthening. As in, spend some months on the wall developing the motor pattern and keep the reps between 1-5 per set. Less than you think you need.
The prep and pushup work all comes from gymnastics, and it's overlooked by a lot of grip people who just close grippers and fo farmer carries (both of which are great, but limited). If you get to the point that you can do tons of wrist and finger pushup variations from the ground in a full pushup position, you will be crushing most of humanity without ever closing a gripper.
In addition, I like hanging. Hanging is stupid-good for soooooo many things, grip among them. Ido Portal has a great series of videos on hanging.
http://www.idoportal.com/blog/hanging
Make aure to explore the rest of the site for more hanging videos. For basic grip strength, just use passive and active hangs, and progress in this order: two hands with feet on ground, two hands free-hanging, one hand with feet on ground, one hand free hanging. Bonus points if you can do all the hangs with a false grip.
Obviously there's a lot more to grip than the stuff I posted here, but I truly feel if you do the stretches and prehab, plus a lot of hanging (daily) and a few wrist pushup variations (1-2 times a week), and you take is slow, you'll be bulletproof in a few years.
I guess the basics turned into a long post anyway, but it's really just the tip of the iceberg, although more than most people will ever need.