Here is a technique I have developed after many years of almost daily brewing. I call it the "30-30-30" (anyone get the reference?), but each step has a lot of room for variance, no timer necessary.
* Mount the AeroPress on a 3dl mug and add ~17g of medium-coarse freshly ground coffee.
* Add 20-30g of hot water to let it "bloom", which releases CO2 and other gases trapped in the beans from the roasting process. Leave it for 15-30 seconds until most bubbles have popped (if any). Very fresh beans should bloom longer.
* Pour water up to just above the 4 mark. I like to spin the AeroPress while pouring to stir it, but you can also pour in circles like a pour-over. Try to make it foamy by having the water break just before it hits.
* Let it drip for ~30s (or until the water level is just above the 3 mark).
* Slowly plunge for another 20-30s.
That's it! The water temperature should be between 85 and 95 °C; any hotter will burn the grounds and lower temperatures won't be able to extract as much flavour.