Your first step will be to download the Wealthsimple Trade app! The Trade app is separate from Wealthsimple Invest (Wealthsimple's portfolio management platform).
You'll then need to open the app, and tap "Get Started".
From there it's just a matter of entering the required information when prompted and agreeing to your account documents! If you're an existing Wealthsimple Invest client you will be able to indicate as such and will be able to skip some of the preliminary questions.