I have't done mine in Canada but I can comment I think because it should be all the same.
I did medicals twice. Never been instructed to not have breakfast.
If you have a UCI number make sure to have it on hand because they'll ask you about it.
Double check info printed on your exam paper, as input mistakes are not uncommon.
The whole thing is pretty straightforward. Asked some questions re your health, some blood pressure and other examinations. Then off to urine and blood and x ray tests. In case the clinic tell you it'll take x days for the results of those tests to come up, make sure to diarize it and call them up to ensure they've sent the results info to CIC.
Other than that you're all set.