The flashing procedure for Bell is the same with Telus and Rogers and AT&T etc.
I prefer flashing with TWRP but I'll walk you through CWM.
Make sure your version of CWM is up to date (through ROM Manager) selecting d2att instead of d2can saves a lot of heartache in the future.
You can download the latest CM10.1 nightly here
If you want the most recent monthly (supposedly more stable but with less features than the latest nightly) you will get this.
Once you download this you'll need the gapps package here
Gapps are the Google apps your phone uses and are necessary.
Place those two files on the external sd Card (if applicable) If you don't have an external card you'll just need to be careful not to format your internal SD. I like to create a folder called ROMS and keep sub folders for each rom and include the Gapps file in each for easy organizing.
Once this is done, reboot into recovery. Press and hold Vol+, Home and Power until you feel a vibration, release the power and it will boot into recovery.
Clockwork Mod is a fairly basic recovery. Navigate with your Vol+ and Vol- buttons, making your selection with the home button or power.
Very first thing you do is do a backup! If you mess something up you'll be able to restore through recovery to your current status and try again.
(Under backup and restore)
Next you are going to select "Wipe data/factory reset" than select "yes"
Than you are going to select "Wipe cache partition" than select "yes"
After that you will go into "Advanced" than select "Wipe Dalvik Cache" and select "yes"
When that's finished you will go back and select "Mounts and storage" select "format system" than select "yes"
Select "go back"
Choose "install zip from sdcard" than select either "install zip from sdcard" or ext sdcard" depending on where you put it.
Scroll down to the folder, select the folder, select the CM10.1 zip first, select yes and let it work.
Once that's done you'll do the same thing for the gapps package.
After that's finished I like to wipe the cache partition and the dalvik cache again (in advanced)
Reboot and voila! CM10.1
Takes a hell of a lot longer to type than to do...
I did find one YouTube video you can use as a guide
If I helped you in anyway please hit the "Thanks" button.
If I really helped buy me a beer sometime!
Canadian GS3 Running
Quantum v3.64 4.4.2
Quantum Core 3/25
0-Day PA Gapps 3/21
3250mAh Extended Battery