If you have trouble silencing the voices of criticism and procrastination, you may want to start or join a Comedy Lodge. The Comedy Lodge is based on the "immersion music method" of songwriting explained in the book The Frustrated Songwriter's Handbook and adapted here for comedy. The Comedy Lodge is a group of people writing humor independently all day long and gathering at the end of the day in an event free of consequences and expectations.
The goal is to create comedy, but also to teach each other to break individual inhibitions and procrastination. You can't get hurt, nothing counts, and you're just messing around. You're just being persistent about messing around. There's pressure, but it's pretend play game pressure. You can't be too critical to write, because the demands of the game require you to work too fast to judge.
The goal isn't to write something funny, the goal is to write something DEVELOPABLE. You're not writing jokes, you're writing "lodge jokes." Write dumb jokes and trainwreck punchlines on purpose. Focus on making your lodge buddies laugh at the end of the day.
At the lodge meeting, all feedback is positive and in the context of developing the joke, rewarding each other's bravery for opening up and taking a chance, what you like about the joke, and how to move the joke forward. THIS IS NOT TO BE NICE. This is to keep the focus on developability.
Then another member (let's call him Bob) reads one of their ideas from their notes. This is when everyone starts talking. Everyone riffs off Bob's first idea and Alice writes down everyone's riffs as fast as possible until everyone's done riffing off of Bob's first idea. The point of this is to riff off the riffs, because this second-level creativity is where you get material that's developable.
Then Bob reads his second idea and everyone riffs again and Alice takes notes until Bob's out of ideas and it's Ted's turn to read his ideas about Alice's joke one at a time so the lodge can riff on each idea. Then it's Bob's turn to perform ONE JOKE.
Loop around the members until you reach the wrap-up time, then keep going until everyone gets an equal amount of performances (except the latecomers who lose a turn). Then everyone picks the best joke each person did and they are honor-bound to perform some version of that joke the next time they're on stage.