i have tiled loads of columns, things to be careful of ,columns often bulge in places due the way they are made,measure round the column to check it. it needs to be plumb as putting a level line on a column that is leanining over will cause you all sorts of problems its good that you are using mosiaic as this will help you if the column does bulge as you can cut the sheets and spread the joints as you need to. prime the concrete, i wouldnt use a level line i would use plumb lines struck down the front and back of the column and put several datum points around the column using a water level then wrap the moasic round the first course using a grout float to flat the mosaic and position it so its meeting up with all my plumb and datum points start at the narrowest point of the column ans its easier to spread the tiles if there are any bulges in the concrete .they are alot easier than you think and once your first course is in you fly it