I get a lot of the backgrounds from Etsy. I just search the particular theme I’m looking for and then I go from there. I sometimes also add more to an image myself and use overlays etc that I purchase through Etsy as well.
So your first step is to find your digital image that you like.
I haven’t used any tutorials I have just taught myself so I can’t help you there sorry.
Then I photograph my subject on a black background or a white background depending on the digital background I have purchased. If it’s a darker background I go with black if it’s lighter white. Remember to make sure your lighting matches your background.
Once you have your image of your subject add it to the digital background as Damien has linked above and mask out the unwanted areas. Next step is to get that size correct so it looks most realistic. Then it’s your levels and skin work. Once that’s all done I add shadows which plays a big role it getting it to look like the person is actually in the photo.
Hope this helps and good luck they are loads of fun to create.