When you buy prepackaged wooden blinds, they may not be the exact width that you need. If this is the case, buy the smallest size that will cover the window. You can cut it down to size. This method is only good for an inch or two on each side, otherwise you will cut too close to the strings. The headrail is normally metal, so you will need to do this project in two parts.
Cut the Headrail
Measure the amount you need to remove from the lift side of the headrail, and mark it with a pencil. The lift side is the end that contains the mechanism for pulling up the string.
Remove the tin bracket at the end with pliers.
Use tin snips or a hacksaw to cut off what you have marked.
Put the end bracket back in.
Bend the top of the headrail a bit with the pliers to hold the bracket in.
Cut the Slats
Press the edge of the folded-up slats against a board or straight edge to line up the slats.
Clamp the slats together, away from where you will be cutting.
Wrap the end of the clamped slats with masking tape.
Measure and mark where you need to make the cut. You will cut through the masking tape to prevent chipping of the wood.
Cut through the masking tape and slats slowly with a miter saw. Remove the tape.