You really don’t maybe you shouldn’t. I just try to compare new things to things I have done in the past and evaluate how much effort more it will take. At work for example: every change is 2 days. You want the little light to be green instead of red? 2 days. You want to change the programming language? Still two days. On average it works out. You just keep stacking successes behind you gradually taking on bigger and bigger tasks. Each one is only slightly harder than what you have already done.