I am going to start on my first potted plant experience and need some soil and draining help.

I was gifted to pots that have no holes, do I drill holes on the bottom or on the sides, and how many. These are roughly 12" pots.

I am going to be planting mums for the fall and even though i am getting decently priced mums i don't want to go crazy on special soils, etc. for mums (sorry). I have naturally sandy soil that drains well here so I wanted to know if I could add a bottom 1/3rd layer of sandy soil and then 2/3 of potting soil (or some other percentage).