This term is derived from the
Coronation street character Liz McDonald, who frequents tarty clothes, suitable for a younger generation, maybe 15 years ago. She wears lots of make up, and her trademark is a gold "L" around her neck.
The term "Lizzin it up" therefore means basically to put on a lot of makeup and/or dress up in a way similar to Liz. However, as of late it has been used to describe any kind of
preparation before going out
on the town.