How do I clean my outside AC units safely?

You can flush out debris such as leaves using a garden hose. Be sure the unit is turned off or you will get sprayed in the face from the fan!

Yuk! Flushing with a garden hose. You can, but I wouldn't until I vacuumed what I could when it's dry.

Turn the unit off at the disconnect.

Remove the access panel.

Vacuum with a wet/dry vacuum.

Clean with something like windex.

Inspect the coils. Vacuum carefully.

If it's really bad, there is professional stuff available to clean the coils.

Consider an AC cover. There is mixed controversy concerning it's use. What you want to do is allow some air into the unit so that condensation doesn't form on the inside with the cover on and turn the disconnect around. It will bee the off position.