I've done several this year, it's really quite easy. Just fill out the applicable sections of the form and make sure you include your employer information with a contact person in Payroll. As you're mentioning Questrade, I'm assuming you are investing that sum into an RRSP. Simply choose whether the deduction will be on salary or lump sum, then enter the amount you're investing in RRSP. Sign and date the bottom. I include a short letter with mine explaining that I will be depositing the money in a self-directed RRSP account with Questrade and invite them to call me with any questions. Mail it to your local tax office... done. No extra forms needed.
You will get a response in the form of a letter in roughly a month. They've always approved my requests and they list some conditions like needed sufficient RRSP room, you agree to pay tax in April if you owe it, and the usual stuff. Basically take this approval letter to your payroll dept and they need to get a copy for their records. My employer makes the adjustment on the very next pay period.
