This short video shows you how to prep and service the sunroof drains on your 2015-2021 VW GTI, but it also applies to the 2010-2020 Mk6-7.5 Jetta/Golf platforms.
The techniques & principles are transferrable to most other modern vehicles where sunroof drains must be cleaned annually as part of your normal vehicle maintenance.
Most modern cars have the drains running in the same basic routes, 4 total drains, 2 in the front A-pillars & 2 in the rear C-pillars, so the only variance will be where your drains terminate under the hood/car, and what tools are needed to get to those drains.
NEVER use pressurized water for this procedure, and avoid using "snack tools", as they can and often will puncture the delicate rubber hoses that comprise the drains.
Having some lint-free blue shop towels on hand and some compressed air for the upper gutter area around the sunroof is both wise and helpful.
Hope this helps some folks out! -The Texican