Sorry to hear it -- I'm sure it's eating away at you. I'm a founder who's had to do it too, in the past. It never gets easier; you just get better at it.

I agree with @steven2012's feedback. To add a few things:

1) I'd recommend you work with your attorneys to draft up a termination agreement that includes a severance package. I'd also recommend you cut a check and include it with the letter. When you sit them down to talk them through the agreement (individually), you can present check to them and ask them to sign it and take the check. You don't want to force them into signing anything, but you also do want to try to get it done cleanly and completely for the sake of the business moving forward. If they hesitate or say they want to review the terms later, or with an attorney, don't push it. But do make the check large enough that it'll be easy for them to sign on the spot if at all possible -- you want to treat them fairly and not have to deal with loose ends later if you can help it.

2) It's extremely important to take the "one and done" deeper cut approach. There's nothing worse than everyone wondering when the other shoe is going to drop. I'd recommend you take the remaining team out to lunch and make sure they know that they are safe -- and you all have to rally together to make the company a success.

As an aside -- the only regret I've ever had is waiting to fire people that I knew had to go. Learning to be more aggressive about moving under-performers out faster has been one silver lining from the whole process, over time.

Good luck. It's a terrible foreboding feeling going into it, but you'll fee a big sense of relief afterwards, and if the employees you're letting go are unhappy in their jobs, they may end up thanking you later for helping them move on to find something they loved more.