To begin with, you need a clear idea of what you want to build, there are some old favourites such as Snow White, and quite commonly we have entries from recent films. A simple idea is to pick on a theme and build around it.
Most people do not own a 20 foot flat bed trailer, but really any size trailer, with sides would do. In fact the smaller the trailer, the easier it is to build! If you have a pick up truck, use of a flat bed lorry, or even a trolley, they can all be adapted to become a float. In fact a child's push chair makes an ideal float for a child to use by simply surrounding it with cardboard and painting it. Once they are decorated, the mechanics of whatever you use will be irrelevant.
Keeping it cheap is important, we wouldn't want anybody to be out of pocket, so before you go to your builder's merchant, look in the garage and see what can be adapted; see if there is anything at work that you could use. Old tins of emulsion paint are the best paints to use since they only need one coat. Cloth, old sheets and cardboard are favourites, as are Christmas decorations. Attach items with a staple gun, glue, and cable ties, but be sure the main structure is secured with screws. Try not to pay to much attention to fine detail, most of it would be missed in a procession.
Rope lights are fast becoming a reliable way of illuminating queens floats, they also require little power to run small strings and can be purchased from most hardware shops.
Christmas tree lights are the cheapest way to illuminate a float. If you can get 12V or 6V DC sets that can run on batteries, even better.
Spot lights are great for lighting up a single feature, but remember that halogen lamps are power hungry, and can get very hot.
Another cheap way is to use shiny materials which will reflect light. Glitter, sequins and reflective paint will catch enough light from a hand held torch and surrounding street lights to illuminated a persons clothing.
Fluorescent tubes are generally impractical since they are so delicate and unless hidden well, can look quite unsightly.
Car spot lamps attached to a roof rack proved ample light, if you have a spare set, its a great way to illuminate a float.
Be sure that your lighting is able to cope in outdoor situations. If it should rain, fluorescent tube circuitry generally fails, and household bulbs pop if water touches them when they are hot.
One of the most important things to remember is the safety of those on the float, and the general public. Make sure that everything is well attached, and does not have any screws sticking out. If children are riding on the float, make sure they can not fall out. Avoid using naked flames on your float, and be sure that electrics are protected from bad weather. If you use a generator, ensure it is stored away from flammable materials.