It's a slow and painstaking PITA to be honest, but I've learnt some techniques with it and I don't do stuff like this often enough to make it worth crawling up the learning curve of a better program.
The trick with photographs is copying the textures used for shaded colours. Paint has no facility to copy an area of colour, just the individual pixel that you click on, so the only way to copy shaded colours that are made of loads of different coloured pixels is to copy and paste an area of it. Use the irregular area copy tool and then flip it different ways when you paste it to avoid repetetive patterns. Then copy and paste some of what you've already done to mix it up even more. Extending shading into narrow areas is a paint but you can do it.
The other thing is to be methodical and copy EVERYTHING before you change it. Paint has no layers or masks like Photoshop, so once you drop Image A onto Image B, Image B is changed forever unless you've got a backup copy.