If you have a Windows PC it should be as simple as downloading a zip file (the correct one for your x50 model), unzipping it, connecting your x50 to the PC and double-clicking a .exe file then follow on-screen instructions. This is not like rooting an Android device, no technical knowledge is required. All PRSPlus does is add some extra very useful menu items to the standard Sony ones.
If you want to do a little research. this is the
main MobileRead thread (its very long) and
this is its Project page. Most people who do install it love it and the developers are active in the main thread. If you later decide it's not for you there is also a PRSPlus uninstaller for each x50 model.
[Edit:] To customise epub fonts a little extra work is required to set up one or more text-based CSS files defining which fonts you want to use. This may sound difficult at first but there are many of us who can help you with your first one as a boilerplate. Once you've got that you can easily create more to use different fonts and/or font-sizes and/or line spacing and/or margins.