What size cutting string do you use and how do you install it on weedeater featherlite plus

The size string is limited by the size of the machine specs and then attachment. I'm guessing 0.65 would be yours. Look at the packs at the store and it will show on the back what will fit what.

If you just by the line and not spool and line then here is some help on a restringing the line.

If the weed eater has a changed head for push in double cut then the "string" will be either in strips or in a roll you can cut to lengths

YouTube - How to Use & Maintain a Weed Eater : Learn How to Spool with a Weed Eater