Hey Dummy, It depends on your coffee grinder. If your grinder has pre-made settings for different grades(Coarse, Med, Fine), you can only grind it down to the finest setting. If it's the kind like a blender, you just throw it in and keep the switch on until it's as fine as you like. You can get it as fine as a powder if you leave it in there long enough.
Either way you still should put it in the mortar & pestal next for final processing.
MM -- I nose what I likes, and I likes what I nose --
Leave it out to dry for a couple of days beforehand and throw it in the grinder. It's all a crapshoot, though. I love the smell of Mac Baren's Virginia #1 as a pipe tobacco but hated it after I ground some to use as snuff.
A compacted tobacco like twist or plug won't dry out enough to make snuff with just a couple of days exposure to the air. You'll do better to cut the rope into small pieces and dry in in an oven at about 120 degrees F until it's crumbly. If the snuff is fine ground enough to suit you as it comes out of the coffee grinder, don't bother with the mortar.
I made a wee bit from some SG #4 twist. Coffee grinder/whirly blade, microwave 15 seconds, rubbed out; mortar/pestle (spoon/bowl). It tasted fine. I need a real mortar/pestle.