I really wish a certain well-known company hadn't appropriated that phrase; it's a good one to live by.
"It's a lovely day. I could go for a bike ride at lunchtime." Just do it. "But I don't have time/What if I get a puncture/I should do x instead/what should I wear..."
Just do it.
I did, just now. Did I go far? No, but far enough. Would I have won a race? Definitely not. How about an award for technique? Nope. But do I feel better for it? Oh, yes. And virtuous too!
Trying to decide where to start on the mountain of work today? Just do it. (Just one thing. Write one sentence, open one document. The rest will follow.)
"I want to write." So write - just do it.
I am - on this blog. I jumped in, just did it, and, like a duck paddling, am keeping afloat.
"I want to start a business." Just do it. OK, this one takes a bit more thought and preparation! But even with as big a step as this one there is always one thing, one small step, that we can "just do." And just doing that one thing makes it easier to keep on going, and eventually taking the plunge to "just do" the really big thing, after doing the lots of little things first, often isn't so much of a huge jump after all.
If we let the un-named company keep their slogan, there are other ways to say this: stop prevaricating; stop procrastinating; stop second-guessing everything. Or, in the immortal words of my family, "Stop poncing about and get on with it!"