The brokerages at BMO and RY are the easiest platforms to use for Norbert's Gambit. Purchase the interlisted security in CAD (USD) and sell it immediately in USD (CAD). This will get you inventory of your desired currency. For the others, namely TD, CM and BNS it's a little more tedious. You'll have to actually call them to 'journal' you trade to the 'other' side (either the CAD side or USD side). Sometimes you have to wait for the shares to settle (T+3 days) before they allow you to journal. There is a lot of material on NGs.
For the latter three brokerages, if you use the CAD side of your account for USD denominated stocks, you're going to get dinged on both sides. To avoid this, convert your CAD to USD via NG, keep it in USD and do everything on the USD side of your account.