Instead of replacing Venetian blinds that are not working properly, you might try to repair them instead. A few materials and a little time may save money if your blinds are expensive blinds.
Lower the blinds and open the slats to the fully open position.
Remove the blinds from the mounting brackets at the top of the window. Lay the blinds out flat in front of you.
Remove the end caps. Remove the cover on the bottom rail to locate the knots or hardware that connects the lift cord and the ladder string to the bottom rail of the blinds.
Disconnect the lift cord connections as they connect to the bottom rail.
Pull the lift cords up through the venetian blind slats so that you can remove any damaged blind slats. Remove the necessary slats and place the new slats onto the ladders of the blinds. If not replacing the lift cords, thread the lift cords back down and reconnect them where they belong on the bottom rail.
Pull the lift cords all the way up to the top of the blinds to replace, if necessary. Pull both ends of the lift cord all the way up and over the pulleys. Then remove the lift cords by pulling them out.
Insert the ends of a new lift cord down through the top and over the top pulley. One end of the lift cord goes over the next pulley and the other end crosses the top to the pulley on the other side of the blinds. Both lift cords need to be passed down through the slats through the entire length of the blinds where they will be secured at the bottom rail. Knot the lift cords or secure into the hardware. Reinstall the cover and end caps of the bottom rail.
Re-hang the blind. It may be necessary to level the bottom rail if the lift cords were replaced. Look for an equalising buckle to adjust for the lift cords when you have the bottom rail even. Trim the lift cords, if desired.