It shouldn't come as a surprise to them if you've been doing your job. You've worked with them for months (I assume some significant length of time anyway), you

It shouldn't come as a surprise to them if you've been doing your job. You've worked with them for months (I assume some significant length of time anyway), you've presumably told them specific steps they need to take to improve their skill level, (not just nickle and dimming specific things that you happen not to like which they may or may not learn from,) which they haven't followed up on, and warned them that they need to do those things or they're going to get fired. "Bob, we've talked about this, I told you months ago that there'd be consequences if you didn't do X Y and Z. That hasn't happened so we're letting you go. We've enjoyed working with you and good luck with your next job." If you haven't worked with them like that, sure, that's a difficult discussion to have without feeling like an ass. You've still got to tell it to them straight, pussy-footing around won't help anything. "Bob, this isn't working out. So, we're letting you go." If it seems upsetting, there's not a way that I know of to sugar-coat not having worked with someone - that seems to tend to make it worse.