1. first you need to open the background (file -> open... -> whateverthenameis.jpg)
2. open the image of the actor (preferably one already rendered where the actor has no background or was cutout from unwanted surroundings) google: "actor's name render"
3. select the actor with the select tool (very top icon that looks like a mouse on the left toolbar)
4. move/drag the render to the top toolbar tab with the background you opened and keep your mouse on that tab (it will automatically switch over)
5. place him wherever you want on the background.
before you can select the actor, if the layer says background in italics you will need to layer him from the background:
right click the layer and select "layer from background..."
lmk if you need further help.