Covering square and rectangular cakes with icing can be a little challenging the first few times you do it. I know it was for me. Getting those corners covered with icing and not gouging them or filling them with crumbs can really be hard. Please click the link if you want to see my complete tutorial for icing any cake.
The first step to keeping crumbs out of your icing is to make sure you have enough icing on the cake. I see my students do it every month... you want to make sure you have enough icing, and you don't want to over-ice it, but the secret is that you can't have too much icing, because once it gets covered, you can always thin some of the icing back off.
Spread the icing until it hangs over the side of the cake like a snowdrift. You want the overhang because you want to meet the side icing with the overhang icing to keep from gouging that top ledge.
Then cover your sides with icing, leaving the corners for last.
To get the corner, load up the back of the spatula, and start maybe 2 inches from the corner, push the icing towards the corner so that you have kind of another overhang (only this one is sideways).
Repeat on the adjacent side until you get this, a big smoosh of icing at the corner-- but no cake showing.
Now, gently, gently smooth out the extra icing, by pushing it down the sides from where it came.