Ready-trimmed leeks need little extra washing, but older loose ones sold with long dark green tops may well have a lot of dirt trapped inside. Wash off any obvious mud on the outside and cut off and discard the root and tough tops.
Slash the leek twice lengthwise from the green tip down to the start of the white stalk. Wash well in at least 2 changes of cold water until all the dirt is removed.
Cut into 3 or 4 shorter lengths, then slice into thin strips. Alternatively, slice whole leeks across in rounds at 5mm (¼in) or 1cm (½in) intervals.