Hi fella,
For washing you need a good lambswool or microfibre wash mit, these are soft enough not to cause any more marks during the wash process.
Use something like meguiars gold class car wash, 2 caps in a bucket will be fine.
If you have a detergent bottle fitting for your power washer, you can pre rinse the car, then soak with some of the car wash in the detergent bottle to loosen the worst off.
Wash using 2 buckets if you can, one with the solution, one with plain water to rinse the mitt out in, this stops you transfering dirt back onto the car and causing marks.
To dry you can use a chamoi if you like, but pat dry or pull the leather off the car with the leather opened out and flat on the surface (if you use an open hose to rinse, you should find the water will sheet of and you won't have too much to dry up)
As for polishing, by hand the best stuff is autoglym super resin polish, its very good at removing light swirls and also contains filler oils to mask any heavier swirls, use microfibre towels to remove excess and buff up the shine, follow this with a good quality wax, this is a personal choice and depends what you like but something like dodo juice is very good, around £30 a pot but it competes with much more expensive wax's for shine and longevity!
Or you could go for a synthetic sealant like jeffs acrylic, these give very sharp reflections and last for months.
Hope this helps mate
Cheers,
Darran.