Camouflaging a person into a background is much easier said than done. One thing you have to consider is your positioning and where you will be taking the photo from to ensure your lines match up perfectly all the time. This is harder than you think because you have to keep stepping back to check your positioning and it’s very easy to draw a line in the wrong place or at a wrong angle.
The easiest way I found was to draw small dots using a white face paint to mark where the lines needed to go and then to fill in using colour. Using a white face paint to map things out is much easier to remove than a deeper colour if anything went wrong.
Another aspect you need to consider wouldn’t be your models position as they would need to stay in the exact same place the entire time to ensure the lines painted matched up to the background. You need to check your model is comfortable and is alright throughout the process. If your model did happen to move it would be quite difficult to match them up again but your model always comes first.
I found this camouflaging into a background very difficult but the busier the background is the easier it is to disguise any mistakes.
Here a few examples I have found in the internet: 
Becky x
hairmakeup.fashion.blog 