Use the point buy system with these guidelines:

All ability scores start at 10.

You have 20 points to spend on your ability scores in any manner you choose but may not go over 18. (Racial modifiers may put you at 20 max.)

Points are spent on a point per point basis. For example, an 18 costs 8 points.

You may lower one ability score to an 8 giving you +2 more points to spend.