There are different kinds of scripts and each kind works in different ways. The most common you will come across are user scripts. A great source for user scripts is https://greasyfork.org They cover what user scripts are and how to use them far better then I ever could on their home page, so I recommend checking it out. The short of it is that you will need to install a script manager for your browser and add the desired scripts to it (Greasyfork will do the last bit automatically for you.) Then all you have to do is go to the target website. Some scripts will effect all websites, some will effect specific websites, and still others will require you to go to a address unique to the script. It should tell you this in the description of the script. If you have any questions on specific scrips that you come across, feel free to ask. I am sure someone here will be able to help.
