you have objectives depending on alignment and map for example:
terrorists in a nuke launch site map would try to launch a nuke within a certain time, and special ops would try to stop them.
in any map, you can win by killing all the enemies, but that makes it less fun.
damage is determined by a special formula:
the playing field is best made from legos or some other stanardised size 3-d building blocks, so that line of sight and distance are easier to measure.
the players can move four directions: north, south, east, west.
they can also, depending on terrain, climb ladders or jump over small obstacles of a size dependant on you character (
each stat(STR, DEX, SPD, ACC, INT) maxes out at 10 and starts at 3. they determine how far you move per turn(SPD+DEX=spaces moved)m, aim(ACC+DEX*5=h;ance of hit, how much can be carried(STR*15=weight limit in lb.) and sneak attacks/ambushes(INT+DEX*2= % chance of success)
you will only need a d100, so it is best to use an electronic die, seeing as d100 is big , clumsy, and hard to come by.
that's about it... change the rules and equasions to suit your prefrences and ability, that's part of what makes RPing better than videogames, you know what i mean?