In the Home Interior Design app, to add objects like doors, windows, models, materials to your plan, all you have to do is drag and drop.

Just select the desired category on the Shelf (for example,  Living Room --> Seating), touch and drag any icon to the right, and that model will be added to your plan.

To add a door/window, you'll need to move it to the wall. If the door is placed on the wall, it will be added. If the wall is too small for the door, then add the door to another wall and reduce its size.

To add a material, you'll need to move it to the object (wall, floor, model, door or window) and to release the finger.