I handwrite each one and follow a general formula, but I tailor each one to the player I'm writing to. I start by saying I'm a huge hockey fan and would greatly appreciate it if they'd take the time to sign. Then I talk about some specific things I admire about their game and how I think said skills will be useful in helping them succeed as a pro. Examples, great shot selection, strong positioning that helps them create offensive chances, smart defensively, etc. Then I finish by thanking them for their time and wishing them good luck in their careers, and I request personalization in the PS.
Beyond the general formula I'll personalize it when I can for each player; for example, in my letter to McLaughlin I wrote how it was cool fo see a fellow PA kid succeeding in hockey and how it must've been cool to be drafted by his "hometown" team. For players like Seeley and Conner Roulette, whom I did feel went way later than they should have, I said that 30 teams are going to regret passing on them. Basically, I try to make it more personal than just saying "I think you're a great player," blah blah.