Determine where you are going to add the circulator pump in the line and mark the pipe to be cut. If you are just replacing a circulator and there is already one there with flanges you will want to unbolt the flange bolts. Once the flange bolts are loose you can remove all but one flange bolt to hold the pump in place. Please note that there may be water that also pours out when you break the flange seal. The amount of water that comes out will depend on what type of line you are on.
If this is a new application and is sure the supply is off. Measure the pump height but be sure to take the measurement with the flanges attached. You will now want to cut your pipe in the place that you will be adding the pump. Once you have cut your pipe plumbing the flanges using your tools and fittings.