You will probably want three strips of heatshrink. They will need to fit over each other shrunk in place so likely need to be different sizes.

Cut the smallest diameter sleeve to 2cm long. Once shrunk into place the white plastic outer will need to fit over this. You might wish to check that this will work, as otherwise you will need to enlarge the hole in the housing (now, not later!).

Cut the others a little longer each e.g. 2.5cm and 3cm.

Slide the largest diameter heatshrink over the cable.

Slide on the next largest size

Push the remains of the strain relief out of the white plastic outer housing. Now slide on the plastic outer part of the magsafe connector. Make sure that you get it the right way round!

Finally, slide on the smallest piece of heatshrink.