Simply click one of the create portal buttons and click where you want the portal to be, in order for a portal to be made. There is a joystick for moving Scratchy, and PC support with arrow keys. ( Yes, I know you can combine arrow keys and joystick to go faster.)
Please do not try to place portals on top of each other, as once a portal is erased (after creating it somewhere else), the other portal will be partially erased too (temporarily). On mobile, do not use 2 fingers at a time, as that breaks the "Mouse Down?" button, which is used for the joystick. Placing 2 portals next to each other may break teleporting mechanic until you move a portal. Tell me any ideas you get that I could add!