DMG pg 169 - Ability Scores, 1. Standard Point Buy and 2. Non-Standard Point Buy.
Basically, you start at 8, then it costs whatever the current ability bonus is to increase the stat by 1, min 1. To go from 8 to 9 costs 1 point, or to go from 17 to 18 costs 3. You have an amount of points to spend however you see fit, usually 15 or higher.
I can attach an Excel sheet I use, of course, you need Excel to use it.
Type in point buy, type in stats, it'll do the rest.