It is pretty easy to do what you want. You have several good Ideas as to how to do this project.
1. - start by scanning the picture large..maybe 250%...do this in the scanner. Iscan directly into PS.
2. - Save file....give it a name and save as a PS doc. Let's Call this file 1
3. - Save file...this time save as a different name ...Let's call this file 2
4. - Print out file 2...to be used to draw on....etc.
5. - Scan the finished or revised copy and call it file 3. scan at 100%
6. - Open file 1 (keep it open)
7. - Open file 3 - - Use the eraser tool to get rid of major parts of the pix you won't need.
8. - Use the marque tool (looks like a perforated box)select area you want to use and copy it.
9. - Go back to the open file 1...Paste ....This will put the driver on a new layer.....When working in PS...select to show layers.
10. - You new layer should be the driver.....an the dialog bow of the layer...you should see a way to adjust OPACITY.....
11. - Set Opacity of new layer to about 20....this will allow you to see through the layer to the base layer.
12. - Now you zoom into the area you want to work on...
13. - BE SURE YOU ARE WORKING ON THE CORRECT LAYER....Sorry for the shout.....but in the layer dialog box..te layer that is highlighted in color is the layer you will be working on.....
14. Make all your fine ajustments including position of the object in the layer......When done increase the opacity of the layer.....In the example pix you posted I would make the opacity about 85%...so that guy won't look like he was added in.
15. - Remember save often as you go and even do save as (give a name) as you go so that if you mess up and you start over again easily.
16 - Lasly......When complete....change the image size back to the origianl size you want...remember we enlarged when scanning.
TRY this and if you get stuck post back.....Yeah I teach Desktop pub in HS.....Take CAre.....Jim S