I was just through there in July. It is well marked. The only thing is that you'll need to find out which gate you'll be using when you're in Montreal. There are TVs all over the place to check. There will be moving walkways too, and a lot of them.
Going from Charlottetown to Montreal to Orlando, we landed, then we had to go through security. You no longer need to pick up your checked bags in Montreal, so you go straight to security with just your carry on stuff. Unfortunately, my ticket had the dreaded "SSSS" on it, which meant I had to go through EXTRA security. My friend didn't and she was in the LONG long and out within 10 minutes at the most. I was at least 20-25 minutes or more. He literally took everything out of my bag and then put it all back in, very slowly. I almost think he was doing it on purpose to see if I'd get angry. I even had to go in the Xray machine.
After security (hopefully without the SSSS for you!) you go through Customs. They were very helpful to me, once I explained that our flight from Charlottetown had left 20 minutes late and then I just spent another 20 minutes in extra security, they let us go through the crew line in customs and they called ahead, and held the flight for us.
The airport is long, I find there are tons of moving walkways, but it's easy to follow the signs and figure out where to go. My sister had only been on one trip by airplane before this trip, and she flew down connecting in Montreal, and found it easy. She is not a very independant person, and she did ask for help once.
I am sure you will be fine!
