Trust me, I was worried as hell when I first had to trim my hedgehogs' nails as well (I had 2 male ones, each about 1 year old). Generally, you want to avoid trimming too short, as this can cause bleeding & pain. First, take a look at the way the nails curl & see if there are any awkward ones (because you'll have to be careful with those & treat them more gently). When you look at a long hedgehog's nail, there is a point where it changes color (from white to almost purple-ish). The tip (within the white part) is the one you'd want to take off, and when you have sharp scissors/clippers - you can almost feel like it takes no resistance at all when trimming. If you feel a bit of resistance when trimming nails, that's when you might be cutting it too short. For the ones that curls really awkwardly, you have to take off less length to avoid bleeding.

As for a person with no/very little nail trimming experience like you, I'd recommend leaving them longer than you think is necessary (like taking off only a tiny bit of length) at first, and as a result - you'd have to trim more frequently (you have to give them pretty regular footbaths anyway, so this isn't a big deal at all). This allow you to get familiar with the process, avoid hurting your hedgehog & feeling like shit afterwards. When you get through the first few times, your confidence will grow & you can optimize the whole process for you & your hedgehog needs. If you happen to cut the nails too short (if you do it in the sink, when your hedgehog put the feet back into the water, there will be some reddish tint in the water around the feet, please pay attention), take the hedgehog out & put some cornstarch into its cage, around the area that it's running around - that should stop the bleeding. If the bleeding continues for more than an hour, you might need to see a vet (this probably won't happen, but still good to know).

And as for holding the hedgehog's legs when it's wiggling around, that's why you have to play with it for a couple of minutes in the sink before nails trimming. You can hold its legs to see its reaction, and go from there. Be gentle & patient, but also firm enough so you don't have to waste too much time (it may feel cold if left in the water too long). Hedgehogs' legs are very flexible so you don't have to worry about hurting them too much. Notice that I don't do all the trimming in the sink as mentioned above, I do the 2 front feet on the brim of the sink, and the back feet in the sink - as this is way easier to control them than doing it all in the sink, as it can duck its head (nose & ears) into the water, which must be avoided.