The easiest way I have found is to buy some ready-roll fondant icing from the supermarket
Then, you can just roll a large circle - big enough to cover the whole top of the cake and down the sides. Cover the whole cake top and sides with non-lumpy jam (warmed up in the microwave to make it a bit runnier. GThen lay the icing over the top and smooth it down. Cut off all the extra at the bottom.
Oh. Just thought. If you do use either glaice icing (the one you have been using) or buttercream you need to do a crumb coat. This is a very thin layer to catch all the crumbs. you allow that to dry and then do a thicker layer of the icing and make it smooth, this will stop the crumbs.
You should do this even if using sugarpaste as it helps it to stick. ( and if like me, think jam is gross)
definately do buttercream icing, just google for a recipe. whip it up really nice, then do a thin coat (which will get a bit crumby, dont worry) and then wait for it to dry a bit and either do another thick coat which will be nice and clean or cover with sugarpaste/fondant rolled out from supermarket.