Finish the ugly version before you make it pretty
It's tempting to spend your first hour making something look nice before you've even proven it works. Resist that. A working tool that looks rough beats a polished, good-looking tool that's only half built, every single time, because looks don't matter if the thing doesn't do its job.
I have to remind myself of this constantly, because my instinct is always to fix how something looks before I've proven it even works. Getting the ugly version working first also tells you clearly what actually needs improving, instead of guessing. You'll often find the parts you thought needed the most polish barely matter once the core function is solid.
Try this today: if you're partway through a project and catching yourself adjusting colors, layout, or wording before the core function even works, stop. Go back and get the plain, unstyled version running end to end first. Save every bit of polish for after it actually does what it's supposed to do.