Use a guitar string winder to tighten/loosen strings in order to correctly tune each string to its respective note.

The usual 6 notes on a guitar are E, A, D, G, B, e, in the order of the thickest string to the thinnest.

Smartphone apps or guitar tuners will help with the tuning process.

While tuning, wiggle each string back and forth to remove slack and prevent slipping so the strings will maintain in tune longer.