When heralds talk about devices, they do not say what the objects on the device are in plain English. Instead, they describe the blazon of the device. Most of this special language is derived from Latin and French. I'll go into a bit more detail on what kind of terms are used in a bit.
The emblazon is the graphical representation of the device. It's the picture that goes on a fighter's shield or a camper's banner outside their tent. The emblazon is what is registered by the College of Heralds, so it's important to develop a good drawing for your submission!
In heraldry, not all colors are colors. Instead they are broken up into colors and metals.
A very basic rul in creating devices is a metal cannot go directly on another metal and neither can a color go on a color. There is a really good reason for this: you need to be able to recognize the device from across the battlefield so you know if that other guy there is a friend or foe. Having the color/metal contrast helps with this a lot. Now, you may be asking yourself what exactly is a metal and how does it differ from a color. Here's the breakdown:
COLORS
Azure
Gules
Purpure
Sable
Vert
METALS
Argent
Or
Even modern logos and signs follow this logic. Think of a Stop Sign. It's red with white writing and border. The Pepsi logo is another good example. The two colors are seperated by white.
Just to add another layer of complication to this matter, there are also neutral fields that can have either metals or colors placed on them. They are called firs. Just as the name suggests, these are inspired by real firs. They are:
FURS
Ermine
Vair
Firs can be different colors than what is shown here, but if you do that, you'll have to specify what colors/metals are being used.
When dealing with blazons, you'll come across the terms fields and blazons a lot. Here are their definitions:
Field
The background of your shield/device
Charge
An object that sits on the field
Ordinary
A type of charge - made up of thick bands of color on your device.
Medieval Sensiblities in Design
When designing your device, you should keep the following in mind:
Charges should be two-dimensional and not look like they're in motion.
If you only have one type of charge, but have more than one of them, they should all be the same size and facing the same direction.
Fill as much of the shield as you can with your charge(s). The three B's might help with this. Make your charges Big, Butch and Bold!
Don't be afraid to put charges on ordinaries. This was often done in period and it looks really cool.
Fields strewn all-over with a single charge in a single color/metal is called a semy. These also look really cool and they're very period. Just think of France's heraldry! In the January illustration to Les Tres Riches Heures du Duc de Berry, you can see the seme of gold fleur-de-lis on a blue background that they are known for. In case you're wondering, that is blazoned "Azure, a semy of fleur-de-lis Or." (Click picture at right for larger image)
Of course this isn't all the rules to creating a device. This page has been designed to give you an idea of things to keep in mind while designing it. Check out the Links and Resources link below for more resources!