ok, in edit mode, go to the page that you want the Galleria gallery instered into, in the area you want to insert the gallery into, select "Add to ...." and if you have installed Galleria Gallery correctly within the dashboard, you shoule be able to find it in the "Add Block" menu that opens on your screen. Once you select it, just follow the onscreen questions to populate your gallery.

I hope thats simple enough for you but if you get stuck just give me a shout again.