Black border...

Hello! I've been lurking in this community for ages and haven't posted here much but I really need help. Can anyone please tell me how to achieve this effect with the black border in these icons or direct me to tutorial that may help:





Icon credit: padabee

I was going to ask the icon maker, but my computer keeps on crashing every time I got to that entry.

Ps. I use photoshop.

Thanks in advance to anyone who can help :D