A monobloc tap replaces two taps (hot and cold) with a single unit from which both hot and cold water is supplied. Your main task here is to adapt your existing pipework to fit the tap tail fitted to the monobloc fitting.
Once you have turned off the water by turning the service valves found on both the hot and cold supply pipes (found on the pipes just below the level of the existing taps) you can remove the existing taps to reveal the bare ends of both pipes. At this point you should fix the monobloc tap into position so that the correct lengths and positions are known.
Check that the pipe ends are the right height to connect to the tap tail on the monobloc fitting and trim them down to the correct height if not. If the service valves are too high on the pipes and need to be cut off to reach the correct height, you will need to turn off the water at the stopcock, remove the service valves and refit new ones lower down the pipes. Remember that you will need to add a section of copper pipe above the new service valves to allow the pipes to be connected to the tap tail. You can use new pipe, or the offcuts taken from the old pipes.
Connect a reducing joint to the top of each pipe (usually supplied with the monobloc fitting, but if not they are widely available from plumbing or DIY stores) to allow the smaller pipes on the tap tail to connect to the inlet pipes. You can bend the pipes on the tap tail slightly to line up with the inlet pipes, but be careful not to put a kink in the pipe. Tighten the large and small capnuts on the reducing joint by hand, and then give them a turn or two with a spanner.
You can now turn the water back on (either at the stopcock or the service valves) and check for leaks at the new connections. If there are leaks, you can tighten the capnuts slightly, or undo them and wind some PTFE tape around the end of each pipe to create a better seal.
