If you have valid visa to the US, Japan, EU (Schengen), Australia, UK, you are exempted for Taiwanese visa.
Take note that valid visas meaning multiple entries that are not yet expired and single entries that are not yet used.
You just need to fill out the online form and print the below travel authorization. Present it at check-in counter in NAIA and at the immigration in Taiwan. (Taoyuan International Airport)
In my case, I never encountered issue passing the immigration nor questioned.