I like to use the Boveda seasoning packs. They work well, but can take a while. If you don't have any of those, or don't want to shell out the cash for them...
1- Take a tupperware container that will fit inside your humidor with the lid closed. Fill the tupperware with distilled water (make sure it not so full that it spills), put the tupperware in your humidor (don't put a lid on the tupperware- this should go without saying, but I've had to clarify before haha).
2- Close up your humidor and then Check the water level every 3 days or so, otherwise leave it alone for about a week.
3-Once the humidity reaches about 80% and is steady there for at least 24 hours, remove the distilled water and add your humidity media.
4-Check it the next day and see where it is at.
(If the humidity drops way below your designated RH, put the distilled water back in and repeat steps 2&3)
If after 3 days it is holding steady at the RH that your humidity media is designed for, start adding your cigars to the humidor.
The seasoning process can take about a week to 2 weeks when done properly. Don't rush it
Cheers and good luck!
Edit: Just realized how big that cabinet is. You'll want to put a couple tupperware containers of DW on each shelf. Also, make sure that everything seals up nice and flush