I generally recommend a ratio of about two parts potting soil to one part sand for a terrarium. I find this mix promotes drainage but still retains enough moisture to keep humidity-loving plants like ferns and moss happy. Watch out for potting soil loaded with fertilizer (like the one in the photo). You don't want terrarium plants kicking into hyper-grow mode and outgrowing your terrarium vessel.
Soil mix with water added
Only mix a little water into your soil mix prior to adding it to your terrarium. Very wet soil will be difficult to add into a narrow-necked vessel and it is always easier to add more water to a terrarium than remove it later!
I'll be sharing an ultra-insider, expert tip to remedy an overwatered terrarium tomorrow so stay tuned!