Covering a cake with marzipan/sugarpaste gives you a nice even base to work with, but it can be frustratingly awkward to do. So perhaps you decide to put icing directly onto the cake and attack it with a spoon and a big bowl of frosting - only to discover  that the frosting is ripping away the top layer of the cake and your icing is full of crumbs!

Lets start with covering the cake with a sheet of sugarpaste/marzipan.

First things first - are you sure your cake can cope with being covered with a heavy sheet of marzipan? (assume sugarpaste or marzipan when I say this). A lighter than air victoria sponge or fragile, layered gateaux will probably deflate under such treatment. A more sturdy Madeira or classic fruitcake will have no problem though.

Next, wait until the cake has cooled - warm cake is fragile, and will likely make your marzipan become too soft.

Now, what shape is your cake? Round? Square? or something more knobbly? Your mileage may vary, but I don't find that marzipan likes being stretched and draped as much as sugarpast, so if you're covering something particularly un-even (some kind of theme cake) I would skip straight to the box of sugarpaste.

The next step depends on how brave you are and how much you want a perfectly smooth base for your decorations.

If your cake has a flat top (you can always slice a bit off if your cake has risen up a lot in the middle), and you're willing to accept a few joins as you plan to cover the cake in some other decoration then I think the easiest way is:

1. Roll out your sugarpaste or marzipan to about half a centimetre thick (about 1/5 inch) make sure it's big enough to cover the whole top of your cake. Use Icing (confectioners) sugar to prevent the icing sticking to your board/rolling pin. *unless* you're using coloured sugarpaste - in which case try a (very) little white fat (from baking shops) or you'll dull the colour
2. Brush the top of the cake with slightly watered down jam (my mum always used apricot, but I don't think that that's compulsory) making sure you go all the way to the edges - this is the glue to hold the marzipan or icing on.
3. Place the cake, sticky (top) side down onto your rolled out marzipan, and using a sharp knife trim round the edge of your cake. Press down firmly, then, carefully turn it back upright (the trick is to put cake board/plate upside down on the bottom of the cake, then lift the whole sandwich of plate, cake, rolling board and turn them over - like flipping a spanish omlette)
4. Now you can do the sides - roll out a long strip of sugarpaste or marzipan, wide enough to reach from the bottom to the top of the cake. You do this in more than one length since your board probably isn't long enough for one strip for the whole cake.
5. Brush more jam on to the sides of the cake - again all the way to the edges - and carefully apply it around the cake. Let any excess width poke up around the top of the cake - we'll trim it off at the end.
6. Press the sides onto the cake firmly but avoiding poking holes with your fingers! - where you need to join, you can either roll the end so it's slightly thinner, or press it with your fingers so that you can overlap the end without causing a bump.
7. Nearly finished! Trim the excess side marzipan level with the top using a knife (much like trimming extra pastry when lining a pie dish)
8. You can use the back of a spoon, or your fingers to smooth the seam round the top of the cake et voila! Your done.

If you're desparate for a perfectly smooth finish all over, or you're just braver than I am, you can go for the more professional draping technique. Warning - this is tricky, you need to be careful and mustn't let the sugarpaste or marzipan dry out!

You're going to need a large board, as you need to roll out your sugarpaste big enough to cover the whole cake and sides in one go - for example if you're doing a 6inch diameter, 3 inch high cake, you'll need a board at least 12 inches wide.

1. Roll the sugarpaste out until it's big enough to cover your cake - remember the thinner it gets, the more likely you'll break it - but conversly the thicker, the more awkward it'll be to mold. Aim for about 5mm (1/5 inch) or whatever your packet recommends.
2. Make sure your rolling pin is freshly dusted with icing sugar (or greased with white fat if coloured) and carefully roll the sheet of icing onto the rolling pin.
3. Bring the cake under your now precariously hanging roll of sugarpaste, and place the rolling pin at the edge of the cake, making sure that the dangling edge touches the plate that the cake is on. Now, carefully unroll the sugarpaste over the cake.
4. Professionals use a little panel with a handle (like a plasters float) to smooth the sugar paste on. Again you could use a large spoon or something similar, but I just use my hands to smooth the paste onto the cake. Starting from the middle, stroke it out over the whole top of the cake, then ease it round the corners and down the sides. This is a time to be careful, as if you make the paste too thin over a knobble it may break.
5. If you've left your sugar paste quite thick, you might find it hard to smooth it over the sites. if this is the case, you can sacrifice some smoothness by making a cut in one of the pleats so you can take out some of the excess sugarpaste.

After either of these methods, leave the cake to dry for a bit before decorating. the sugarpaste and the marzipan will get a bit of a crust so they can handle the slightly rougher treatment they'll get when you take the next steps.

Don't be disheartened if you get cracks or crumbs - these techniques take a bit of practise. Until then, remember that you can always make sure that you drape/ice some decorations over the imperfections so no-one can see them.