Mow the existing grass very short
Rake the area hard to expose and loosen some soil
Fill any holes or dips with 50% soil, 50% sharp sand mix.
Oversow with a good quality seeds mixture containing dwarf ryegrass and creeping fescue
Lightly rake to mix soil and seed together
At this point it will probably look pretty sad, but don't panic

Mow on highest setting for 8-10 weeks
Feed well with a good quality multi-purpose fertiliser
Then reduce height on each subsequent cut down to your desired leaf length.
You will then have a good looking, very hard wearing utility lawn.