It's easy. You can leave the hoses connected if you wish. The only thing to be careful of (assuming you have a seperate thermostat and housing, and not one unit) is to make sure that the thermostat is seated in the head when you put the housing over it. If it is cocked, or out of position, you could crack the housing when you tighten it down........been there, done that. :P
What I do is put a little grease, or vaseline, on the backside edge of the thermostat, and also over the front side and oring. This will help hold it in there, and it won't hurt anything. Make sure the oring is seated correctly as well, and it should stay put, unless you bang it with the housing trying to align the bolts.