Hi JoyN23
I went on this trip last February. I wore a goosedown jacket that had a hood and that kept me warm. I also took a woolly hat as you don't want the hood up unless it is really necessary. As I do a fair bit of walking at all times of the year I also have some lined trousers so I avoided wearing longjohns (I did take some just in case). Take a good pair of gloves as well.
On my feet I wore hiking boots with one pair of thick socks and most of the time my feet were fine. The only time my feet felt cold were the day we left Jasper and spent some time at Pyramid Island. It had been -35C overnight and although I have no idea what the temperature was while we were there, the coach driver tried to clean the passenger windows with pure antifreeze and it froze on the glass.
I would also suggest you consider buying ice grippers to put on your boots - you will feel a lot safer walking around on some of the icey areas. One of the ladies in our group fell over on the first day and broke her arm and was flown home a couple of days later.
Having said all that I thoroughly enjoyed the trip, rarely felt that it was bitterly cold (even at -10 or lower most of the time) and the scenery is magnificent.
Enjoy your trip.
PeterG