A missive for you from L, cakemaker extraordinaire, and maker of the Bayeaux Cake, in response to hungry_pixel's questions:
I started with a simplified cartoon of the soldiers and horses on paper and then traced them onto greaseproof paper the size of the sides of the cake. I used sterilised glass headed sewing pins to pin the pattern to the iced cake , making sure to put the pins through a drawing line. I used the longest pin I could find (easier to hold) and transferred the design onto the cake by carefully pricking through the outlines at intervals. Then the fun begins. I used a mixture of food colours, some powders, pastes, and liquids. Using a fine artists paint brush, kept for cakes only, and a clean white plate I mixed one colour at a time as you would water colours. I tested the colour on white paper first and using the cartoon as a guide began filling in the areas between the dots. Two sessions later the painting was complete. I then used coloured royal icing to pipe the outlines of the soldiers and horses. The writing was done freehand and was meant to look slightly wonky.
I have never been on a cake decorating course and don't believe you need to. There are plenty of good books out there to refer to. Buy a piping set, I started with a piping tube (Tala), and now use nylon bags with separate nozzles. Make a chocolate cake, or cupcakes, whip some cream and pipe swirls or stars on the top. In fact you can practise piping lines and stars etc. on the top of a plate. The first serious cake I made was a christening cake for C. This was successful and since then I've made wedding cakes, and anniversary cakes for family and friends.
I'm sure there are courses available through the internet. All the information I use has come from reading different books on the subject, mainly from the library, I actually treated myself to a cake decorating book this year, so I now own two books . Magazines also feature cake decorating occasionally and usually are worthwhile keeping. Fashions dictate how fancy or plain a cake will be decorated, nowadays many wedding cakes are minimally boring, but easy to do. Give it a try.