Use a tape measure to get the width of the window from left to right. Write this number down for your width measurement. If you are concerned about light coming in on the sides, you can add an additional three inches for each side by adding "six" to your measurement.
Stand on a ladder or step stool so that you can get the length measurement.
Measure from the length from the top of the window. Curtain length measurements usually go to the floor, but you can stop at the bottom of the blinds if you want. Take the measurement and add 3 to 6 inches to it so that you have room for the curtain rod. The amount you add is up to your personal preference.
Get a standard curtain rod and curtains, such as panel curtains, that fit the length and width measurements you took. You will likely need to buy two curtains to cover the blinds, which means you should divide the width by two to determine the size you need for each curtain. However, if the window where the blinds are located is extra wide, you might need more than two curtains. To determine how many you need, divide the total width by the width of the widest curtain you can find. The curtain rod should be the width you took down.
Install the standard curtain rod above the window with a screwdriver and slide the curtains onto the rod. Standard curtain rods are made so that you can insert the rod into the sleeve at the top of the curtain. The ends of the rod have plates with holes for screws for easy attaching to the wall. Usually the screws comes with the rod. Once you have the curtains on the rod and the rod on the wall, you should be able to pull the curtains out across the rod and fully hide the vertical blinds.