You could dilute a sample of pure methanol.
The easiest way to prepare an 80% v/v solution of methanol is to dilute a sample of pure methanol with water.
To make the calculations easier, let's assume that you want to make a
If your target solution has a total volume of 1.0 L, then you can use this formula to determine how much methanol it must contain in order to have a percent concentration by volume of 80%.
This means that if you take 800 mL of pure methanol and add enough water to get the total volume of the solution to 1.0 L, the resulting solution will have a percent concetration by volume of methanol of 80%.