I do this route often. Just take your Delta confirmation number and plug it into the Westjet online check-in. You'll be able to select seats there.

A few things to know:

1. By checking in on Westjet's site vs. Delta's site, WS takes control of your ticket.

2. The good news is WS has no change fees, so if you're on WS paper, there's no rebooking fee. Just walk up to the counter, and they'll pop you on the next flight out.

3. The bad news is if you do a WS controlled ticket, during a weather event, you are at the mercy of WS. You have to call WS to relinquish control of your ticket to DL for a DL agent to rebook you on another flight. This is not trivial. The DL system occasionally seems to be able to handle this automatically, but it has been hit or miss for me.

4. WS cancels very frequently, esp. early morning out of LGA. DL is way more reliable in/out of JFK despite a smaller and older plane. On a DL controlled ticket, they will rebook me YYZ-JFK if YYZ-LGA is not possible. On WS, I'm stuck.

5. WS is very seat change friendly. The flights are rarely ever full, so a trip to the desk will get you a better seat even if you're on DL ticket. Don't sweat it if you check-in on DL and don't have a seat assignment.