The problem with this nice piece of advice is that you never know if you're low or high. Bitcoin at 6000 could be low, but it also could be high. Selling at 10000 could be high, but it also could be the new higher low.
That's why the best way to counter the uncertainty of not knowing if the price is under or overvalued, is to simply dollar cost average your entry points. I haven't done anything else in the last years.
It's impossible to find out the right moments to enter, because today's "low" price might turn out to be tomorrow's peak that people wish they would have sold. This market has no mercy for its participants.
People quite often go wrong because they assume crypto is cheap when it has gone down with 50% in a matter of months, but don't understand that it can go down even 90% in some cases.
XRP as one of the largest coins went through a +90% correction from top to bottom.