It's do-able, but please do it right.
Tools: A pump spray bottle (or recycled 409 cleaner/ Windex/Etc type of bottle) Bleach, Water, New Pollen Filter.
The simple answer is to spray a pump spray bottle with 50/50% water/bleach solution and let it sit until it dries. SPRAY EVERYTHING. Seats headliner, Carpet, Trunk Carpet, Vents, Under the dash board, as far into the dash as possible from underneath, Remove the trunk carpet. Spray it front and back, the spare tire wheel well, Remove the spare tire, clean out all the gunk in there, check for water accumulation. Is there water or a lot of condensation? You have a leak in the car. Spray inside the air vents, CHANGE the pollen filter, it's full of mold spores at this point
The more complicated "if you don't want to get sick from mold" answer is to start by finding where the water that caused the mold/mildew is coming from and make sure it's not still coming in the car. Sunroof drains may be clogged. Water (or water marks) on the rear floor or in the spare tire area? Yep- Your sunroof drain is clogged.
Also make sure the door and window seals aren't leaking. High pressure automatic car washes are good for this.
Cleaning technique- "Do it right and don't get sick from mold" method:
1. Remove the front seats (you Tube)
2 Remove the carpet. Clean it outside the car. The carpet has mildew in it, not just the seats.
3. Flip the rear seat lower cushions forward. Clean. Spray the bottom of the seat cushions with the 50/50 solution and let it dry. Flip the rear seat back down- Spray with solution.
Remove the seats, carpets, and the vent tubing and clean each surface with a 50/50 bleach water solution (You Tube is a great source for VW "How To's". Rear floors are catch basins (on any car) for water from open or leaking windows. Check there.
Flip the rear seat cushions up and flip them forward. Spray under/behind them and the carpet that is there as well.
Step 3: Vents and AC System. The vents pull apart, take this time to spray/ clean inside them. Otherwise you may just be blowing mold spores on yourself with the AC.
Change the Pollen filter on the AC system.
Step 4. Take out the trunk carpet, spray with solution, and maybe even shampoo it. Leave it to dry in the sun.
Leave the doors and trunk open for several hours to properly dry the car. Open windows (only), wont allow proper air flow to the carpets.
Avoid using a smell blocker like Fabreeze as it only masks the smell, and wont kill the mold/mildew.
Good Luck!!