its really easy. I can post pics if you want. I don't know how the filter is constructed in the Jetta, but my WRX and Celica were the same so lets assume the Jetta is as well.
1) Go get the biggest and best furnance filter from your local store. (3M purple ones are really really good, around $15).
2) pull cabin filter out of car.
3) Put cabin filter on furnance filter and use scissors to cut the furnance filter to the correct size.
4) put new filter in car.
5) throw old filter out
If you do it right you can get 3-6 filters from one furnance filter. It works out to be around $3-5 per filter and the filters are usually better than the OEM filters (in my experience at least). I have been doing this for year and it has saved me a fair bit of money.