Use the mouse to interact with the character on screen. You can change between your hands, mouth, and dick using the menu in the top right.
While interacting, you can hold SPACE to record your actions. When you let go, the game will continue doing whatever you were doing, on a loop.
Several sex toys can be accessed from the Toybox menu in the top left. Drag them onto the character grid to decide where to put them (or drop them into the tool slot at the bottom if it is a toy that is used as a tool).
You can customize your penis from the Toybox menu.