Years and years and years of using spreadsheets. I started on LOTUS 1-2-3 in 1984 graphing experimental data for one of my engineering professors. I had to teach myself the software and after a while you get pretty good at it. Also, if you have any proficiency at computer programming (C, Fortran, Basic, etc.) you understand a lot of the concepts behind the functions you're using.

Mostly though, it's just years of using the software. A short course can help get you going too. You can at least see what's possible with the spreadsheets even if you can't exactly remember how to do it.