Thats the roller i was on about.
Unlike in the picture, ross is actually doing this badly, shame on him, keep your arms completely straight, this isolates the forearms. DONT BEND YOUR ELBOWS
You attach a weight, start with something like 5kg, roll it up to the top and back down again carefully, not letting it drop, and not too fast, its not a race.
Dont let your arms drop below parallel either. So you do this one first of all and build up to 3 times, youll need a 2 minute rest or so between sets. Then gradually start increasing the weight, something like 10-15kg is a good goal.
You can do this 3 times a week. So up and down 3 times, its very hard, if its not hard enough increase the weight. You must remember to increase the weight fairly regularly in order to progress.
with the ball do static holds with each finger for 10 seconds each, you squeeze the ball as hard as you can between one finger and thumb, one at a time for 10 seconds each. Do this 3 or more times through, just build it up.
In addition you can get heavy duty grippers, look online and you should find some that are higher poundage.
You can do this for max reps and try to beat your last number, there are things for this online as well, you can do things like 25 time arms in front, 25 times arms above your head, 25 times arms out to the side and then rest and repeat.
Stuff like that you get the idea.
If you combine all this plus hitting pads and bag you will get the strength you need in the hands and arms, but build it up gradually.
You also need to train your body really hard, google rossboxing and read the articles on there, you wont get better than that.
However, and this is my opinion, id recommend a regular bodybuilding type muscle building phase for 3-6 months to start with to slap on some solid muscle. You then need to work functional muscle, i like this approach, do your own research though and decide for yourself.