Firstly you must remove the old drain and prepare the new pop-up drain stopper. This is achieved by first removing the trap, using pliers (channel type preferably). Follow this up by removing any old pop-up systems that are in place. Use the same pliers as before or a wrench on the mounting nut to remove the old drain. Make sure that the sink is clean before you continue. Disassemble the new pop-up drain stopper and ensure that the new mounting nut is all the way down the pipe.
Step 2 (Optional)
This step is only needed if you are having trouble connecting the rubber section to seal to the bottom of the sink, if this is the case, apply some pipe joint compound to the rubber section of the washer, this will solve any problems with sealing the new pipe to the sink.
Apply either foam, silicone or putty to the underside of the top piece of the drain stopper, underneath the chrome ring, further aiding with the seal.
Slide the new drain through the opening under the sink as high as is possible, while holding the drain in the position, fit the aforementioned chrome ring into the sink. Screw the chrome thing to the drain, this only needs to be done by hand as it doesn’t need to be too tight.
Back underneath the sink, tighten the uppermost part of the drain while holding the drain in place, ensuring that the nut is facing away from you and towards the back of the sink. Make sure to use pliers and make the drain just tight enough that it cannot be moved by hand, but try not to go any tighter.
Place and then push the stopper into the drain, ensuring that the hole is facing away from you. Go back under the sink, with the ball on the inside of the nut, push the nut onto the pipe, push and pull the rod to ensure that it is working properly in the sink. Reposition accordingly and once completed, use your hand to tighten it into place.
Keeping the stopper open, feed the lever down through the hole next to the taps, connecting the lever to the rod below. Attach any straps and/or screws to the lever and ensure that it is tightly connected.
Connect the lever rod to the vertical drain strap, connect the trap to the pipe and proceed to fill and drain the sink in order to check for leaks.