Plastic, flexible leaf rake. Something you can scoop the leaves up with, but not damage the liner. Once you've got everything you can out by hand, use a shop vacuum to suck out the rest of the small stuff.
Then scrub the whole inside down with a 50/50 household bleach solution (a brush on a broom handle helps) then rinse with water and drain. Fill it up and get to work balancing the water chemistry.
Get dirty, get it all out carefully and start clean.
EDIT: I also like the 1 foot of water and leaf rake idea!