How to 'just do it' and not stay on the sidelines?

Like stock investing, just accept that you'll never be able to buy at the bottom and you'll never be able to sell at the top.

Once you've got that down, you'll be fine. Regularly adding to your position, if you're confident in it, is also recommended.

Finally, start small and add over time. There's no rule saying you need to commit a massive dollop of capital when you first initiate a position (in BTC or stocks).

I was in the same position before.
First, i think you have to be convinced that crypto is an asset class you believe in and may potentially give you super returns relative to risk. This means you are ready to buy because you have conviction. Not because of hype.
Second, get your account set up and funded. This is very important imo because crypto is very volatile and price changes very quickly. If you wait till the time is 'right', then start funding and learning how to buy. You will miss the boat and start the process of waiting all over again. Then the FOMO repeats.
Third, set a trigger to buy. For example, if btc or eth crashes 30% from its recent high, I will buy a certain x%. If it falls another 20%, buy another y%. May not be wise to go all in, but enter in tranches.
In essence, take action. Buy a little cryptos to test out, it's a learning ground. Get used to the fees etc. :)

Advice to buy a little with the amount you are ready to lose first to try it out. See how this thing will work for you. Once you get more comfortable, then you add in more bit by bit.

Read up more about Blockchain and BTC/ ETH to see how they work to have a closure to all your worries.

Once you get more convicted, look at BTC or ETH's 5 days / 10 days / 20 days moving average price chart to see the future buy entry opportunity.

Personally I bought BTC at price 19K USD, 30k USD, 52K USD.

Thewokesalaryman advises us not to have more than 5% crypto as our portfolio. Hence I calculate how much that 5% is, and never go above that value in terms of capital put inside crypto. If it goes down, not the end of the world. If it goes up, gains aren't sky-high but at least it's free profit.