Always easiest to start with the shell fit. Almost every brand/boot uses one shell to cover a whole size and a have size (with a thicker boot board to take up volume in the smaller of the two sizes.) Different brands break their shell sizes differently, though. Some break on the whole size (i.e. one shell covers 26/26.5, another 27/27.5, etc.), and others break on the half size (e.g. 26.5/27, 27.5/28, etc.) So a mondo size in one brand, or even one model within the same brand, is not necessarily equal to another.
Pull out the liner, and put your foot into the shell so that your toe(s) barely touches the front of the boot. Then look at how much space there is between the back of your heel and the shell. About 1 finger of space will be a tight fit, 1.5 fingers a medium fit, and 2 fingers a looser fit. Less than 1 means the shell is probably too small (or you're a racer), and more than 2 means it's too big.
There are lots of other nuances to look at when it comes to fit, but starting with the proper shell is 90% of the battle.