War board games run the gamut from simple strategy games to detailed simulations. Before you make a war game, decide just what you want to end up with. Do you want a game for casual enthusiasts, or one for serious gamers? Do you want a quick, easy game or an all-day campaign? Create a rough draft with temporary game pieces and a sketched-out paper game board to test. Only invest in nice miniatures and create a finished board once you have ironed out all the kinks.
Figure out your scale. Decide if you want your war board game to simulate a single battle, a war, or an ongoing campaign. Look at some games that work on the same scale you want to work with. For example, a game like Warhammer simulates an individual battle in a fantasy universe; Axis & Allies simulates World War II; and Risk simulates an ongoing campaign for global domination.
Come up with a setting. Do you want your game to take place in a particular historic setting, in an imagined future, or in some fantastic world? Part of what makes a good war board game is the fantasy aspect. By keeping a particular world in mind, you can make a game that is more compelling.
Determine with unit types. For a simple game, you could have one type of infantry unit, and one type of artillery. For a more complicated game, you can have archers, cannons, flying critters, horsemen and however many other kinds of combatants you want.
Create rules for each unit. Decide how many spaces it can move, how far away it can attack, how much damage it can do every round and how much damage it can take without being destroyed. You will probably want to base everything on 6-sided dice rolls, since this is the most common way to decide things in a game. A hypothetical unit of archers might be able to do 1d6 (1 roll of the 6-sided die) points of damage per round to any unit within three squares of it. It could move two squares per round, and could take 15 points of damage before being killed.
Draw a map. The easiest sort of map to use is a hex board. A hex board is a board made of hexagonal tiles. Movement is measured in the number of tiles a character can move each turn. Print out hex paper using the link below, then draw terrain on it. You can create solid walls that units can't move through or shoot through, rivers and other impassible obstacles, and other sorts of terrain. Create rules for each kind of terrain. When you have your map completed, mount it on a piece of poster board.
Come up with any other rules you need. You might have resupply rules which tell how many new units a player gets each turn, bonuses which give a player special advantages if he captures a certain part of the map and requirements for victory.
Get some miniatures to play your game with. You can use coins, game markers, or scraps of paper to test the game out. However, eventually you are going to want miniatures to make your game more realistic. Go down to your local hobby shop and find figurines that are the right size for your game.