Potions is the very first activity you do in Year 1 (and likely the game). It's pretty similar to how potions worked in Half Blood Prince. Follow the instructions the game offers you (it's quite simple really) until the mix turns the color in the corner (if it's showing red, mix until the color turns red). Pretty much just keep doing the motion slowly until the color changes and put your arms at your sides when it does. Keep at it until it unlocks. As long as you're accurate and a tad quick about it, you should have no problems unlocking it.