Youtube is your friend. There is a walkthrough / tutorial / tear down for virtually every phone out there.
Must admit, I have been involved in a business recently that does just this and to be perfectly honest, there is little to no money to be made if oyu want to providing a decent service.
Having said that, if you are happy to take a chance on cheap Chinese components (screens in particular) and can get the actual replacement of such a screen down to a knack, you may be able to make a bit here and there, but you certainly wont be able to retire early on it.
Take iPhone 6 screens for example. You can source a cheap Chinese screen for less than £20, but to be competitive you need to be able to supply and fit for £35 or less (a local "shop" supplies and fits for this amount). If you are good, you could do the repair in 15 mins, so £15 for 15 mins work is not to be sniffed at. Well it would't be if you could guarantee 4 repairs like that an hour and unless you have a shop, or can reside in one, have decent foot fall and know that there are no other competitors around you doing the same, then you might be able to cover your costs and have enough for a beer or two on Friday. If you are looking at doing this outside of a retail outlet, my advice is to forget it as you simply wont be able to make the money. There are many cheap while you wait services available in almost every high street now, so you would have to be offering something quite exceptional to even get potential customers to even consider you.
The only real way of making money in this field (seeing as the pop up shops are undercutting each other in a race to the bottom) is to provide a more upmarket service only supplying OEM quality parts. You then market the business as a elite service and play on the quality of the components you are using, and maybe offering a longer warranty to offset the higher prices.
Its a lot of effort for very little return, unless of course you are in the only part of the world where this market has not already been saturated.