Imagine you want to install the very experimental and pre-pre-alpha beegis geonotes inside my current JGrass installation. What would you do?
Nothing easier than that:
1) in the help menu go on software updates -> find and install:
2) select on search for new features and go next:
3) push the New Remote Site button and enter the update site you which to install the plugin from. In this example I will use beesgis-extentions update site which is here:
4) after clicking OK and finish, a list of available packages supplied by the update-site will appear. Select what you want to install:
5) next you will be asked to agree the licenses and so on.
6) if the packages are not signed, you will be warned and prompt about installation. If you are sure, just press install all.
7) you will be asked to restart JGrass. Do so and after restart you will find the geonotes layer in you mapgraphics and the geonotes tool in the info category tools.
The update is done. You can enjoy your new plugin.
To be honest there is one more step to go, since without the right database and database tables there is nothing to be done.
So get the database zipped from here and unzip it into some folder of your choice.
Now let's tell jgrass where the database is. Open the preferences, (Window->Preferences) and under HSQLDB server point to the unzipped geebisdatabase server:
Restart JGrass and the game is done.
PS: if you instead are interested in creating an update site for others, have a look at this nice article from the eclipse wiki.