For finger / hand / wrist training to prepare for claws, you'll need to keep the following in mind:
1. Finger strength
I'm not very familiar with Judo, but I bet this is roughly the same as there, as the basic is the same: Grab and don't let go. You can train finger strength in several different ways
- Fingertip pushups: They're exactly what they sound like: Pushups on your fingertips. If you've never done these before, start easy. There's no shame on starting them standing up, against a wall (hell, that's how I started, and at one point, I could do them with my feet way up on the couch's back rest) and slowly work your way down. Don't overdo it. Let your fingers gradually get used to it. Once you're comfortable with doing regular pushups on your fingertips, you can also start doing them on less fingers (8 instead of 10, 6, etc)
- Stressball squeezing: Stressballs or tennis balls, whichever you got access to. Just grab them, put your fingertips onto the surface, and squeeze the bejeezes out of that thing!
- Grip strengthening thingamajingy: I honestly don't know the proper name for it. It looks like two handles and a curly ring at the top. You grab it in your hand, and squeeze. It's basically a more advanced version of the stressball squeezing.
- Tiger claw: It may sound silly, but a great way to train your fingers for claws, is using those claws. Especially tiger claw is good for this. You bend your fingers, pointing each fingertip forward and make them as firm as possible. If you're not used to it, you'll quickly feel it pull in your fingers, hand and maybe even lower arm. Don't give up at this point. Bite through it and keep those fingers firm! Eventually, your claw will strengthen
- Hitting beans: This is a more traditional type, which is shown almost every time the Shaolin temple is shown. They're standing in front of a jar filled with beans and slam their fingers into it. The hits basically strengthen the bones. You can also take eg. a sock, put a plank in it, and fill it up with foam or hankies or such, to soften it a bit, then loosely hit with the back of your fingers and the front. Or you can buy "iron beans" or use gravel (DISCLAIMER: Don't use gravel when starting out! That's for highly advanced only!)
2. Wrist strength
Wrist strength is more general, and requires to be very careful! If you do this poorly, you'll be lucky if you're out for only a few weeks!
Back wrist pushups: These are regular pushups, but instead of putting the palms of your hands down, you put the backs of your hands down, and curl your fingers up into a "bird's beak" (place all fingertips around the tip of your thumb). Be EXTREMELY careful with this! As always, form matters, but tenfold so with this!
Leopard claw pushups: You make a leopard claw. It's pretty much half a fist. You put your fingertips on the cushions of your fingers, and close your thumb at the side. Then stand in pushup position, with this leopard claw facing downward. Best is to start on your knees and just stay there. Once you're comfortable with that, just stand in pushup position (so basically doing a high plank on those claws). Once you're comfortable with that, you can slowly start doing pushups. This also trains finger strength, but will have more effect on your wrists, so it comes here.
Good ol' punching: Just regular punches: Jabs, straights, hooks, uppercuts. They all create impact on the wrists, and thus are practice for those as well.
3. Lower arm strength
For lower arm strength, any lower arm exercise can work. There's such a wide range of exercises you can do with either dumbbells or barbells for this, I'm not gonna list them. I'm actually only going to mention one, which I think is relevant for specifically this, and not just general lower arm strength.
- Explosive hands: This doesn't have a name that I know of. It's just an exercise I figured out, which helps wonders. You make a fist, as tightly as you can, open it quickly, and close it as tightly as you can again, straight away. The more explosiveness you put in this movement, the better. First time will be nothing. Second time, nothing still. You'll feel its effect if you do it for a bit longer, like a minute or so. It's a perfect exercise to do while you're waiting somewhere, or are taking a walk (both while nobody's watching, cause, y'know...)
That's all the main ones I know of. If anyone else knows more, I'd actually be very interesting too. Finger / grip / wrist / lower arm strength is of such high importance in so many different styles, it'd be interesting to hear how eg. judoka, jiujitsu or aikido practitioners train this.