I am not sure that there is a tutorial for this as such, but, hey, I may be wrong. A good add-on will have good instructions about how to install it but, you are right, they assume a certain amount of knowledge.
In general add-ons are composed of a set of files that you upload by FTP to your server. The files either replace existing files or are new files that are added.
If you download a add-on the first stage is always to un-zip it.
Then have a look for a file that is the install instructions. They will hopefully help a bit.
Again, different add-ons are a bit different but in general the files should be arranged in the same structure as they need to appear on the server. So, once you have un-zipped it have a look at the files in the add-on folder. If you see a file that is in the 'includes' folder in the un-zipped add-on then it is going to need to go in the 'includes' folder on the server.
A good add-on will have the same structure as the folders on the server. This means that, with a good FTP program, you can simply put all the files in one go into the root of the store.
The root for the shop is the folder/directory that holds all the files of the store. It might be https://www.mydomain.com or it might be https://www.mydomain.com/shop - it just depends what you have called it. But it is simply the folder / directory that hold all the files.
I hope that helps a bit. It is confusing to begin with when working with websites but will soon make sense :-) If you say which add-ons you are thinking off then people may have more specific advice.