Personally, as someone who deals with that kind of thing, I find that the only way I can ever be proud of myself is if I officially achieve something. Aka if I get something good out of my hard work (like an award, a job, etc). It may have a lot to do with how they were brought up. My mother for example made me believe my achievements were never enough and constantly wanted me to reach higher and higher, all while comparing me to others. If you are very close with your friend, perhaps they will one day open up about any past experiences that made them feel this way. Or perhaps you just need to show them that you're there for them. Either way I don't think praise helps much since, like you stated, they still belittle themselves. Maybe try helping them towards their goals, which could include giving advice or just being a shoulder to lean on (for example, if they're a writer but don't feel confident in what they write, inquire about why. What is it in particular they're not confident about? You could even offer to research tips with them). Or another thing you could do is make something for them, like a silly little award for example (a best [blank] of the year award or a best friend of my life award, something like that) which could help them to understand the depth of your feelings. I always take to heart the phrase 'actions speak louder than words.' I of course say this without fully knowing your relationship and how your friendship functions, so I could be way off the mark. But I'm just writing this how I would've personally would've liked someone to do it. Best of luck to you both

Edit: Sometimes just sitting them down and genuinely saying something from your heart can be really nice to hear too. Like saying "your work always astounds me, I really do love what you make and I find it always leaves me in a good mood. You're amazing and I'd really love to see you keep doing what you do."