To invest in any asset, first make sure you have enough balance in your account (at least €10).
From the main Invest page, you can browse all available assets.
After choosing an asset, press the briefcase icon on the right side of the asset box.
You can then enter the specific amount you wish to invest or select one of the preset amounts (€10.00, €50.00, €100.00, etc.).
Once you have chosen the amount, press the "Invest" button.
A pop-up screen will show a confirmation notification (you can turn this off if you wish). Once you press "Invest" again, your investment is complete.