Installing boards (and indeed any addon to UECIDE) is very simple.
First open the Plugin Manager, which you will find in the Tools menu.
Next you should press the Refresh button. This will ensure that your package list is up to date.
Now find the plugin you want to install. If it's a board you want to install then you will find it in the Boards section of the tree. This is fairly well organised by board manufacturer.
Finally git the Install button. Your plugin (and any pre-requisites) will be installed. The final "Installing blah... done." message will be the plugin you have chosen. When you see this you know it is finished, and you can close the window.