Turn off the supply
Unscrew the knurled top bit
Take out the screw inside
Pull off the handwheel
Unscrew the shroud
Unscrew the valve and remove from the tap body
Replace the washer
Put it all back together again
I would just say to start this early in the day. Taps have a variety of washer sizes and types. You need to get the proper one. If you have a good DIY shop near you could ask in there first for the washer you need. Otherwise you may have to take the old one to the shop to get the correct item.