ok...start a new space map and in the map context information box do this
faction owner=rebel or whatever i guess
map name=whatever you wish
then go down to the space dimensions and change the space width to say 20000 just becouse we want some space..lol it will automatically change the height too.
now you got a big emty box
in the left pane clik on SPACEPROP and pick a backdrop like alderaan, its the first one.
with it highlighted move your curser over the big emptybox on the right and hit shift key and then leftclik mousebutton to drop. if you got a mousewheel you can zoom in and out with it now to see the borders or your map with alderaan in it.
hold the right mousebutton to scroll around your map.
the rest of everything else is pretty much like that for a space map.
i hope i dont get in trouble for a big post but lets go ahead and make it playable.
in left pane scroll down to starbase and pick the skirmash empire and rebel bases and put them in the map the same way you did alderaan.
take note that things will fade in and out on the map.
make sure to put in some spacebuildable mineral_extractor_pads around your map too
now goto marker in the left pane and find player_0_spawn_point_marker and place it next to the rebel base. in the left pane towards the bottom is "player owner", make it rebel. now do the same with the empire but use the player_1_spawn_point_marker.
ok you just made your first map now just fiddle around and add whatever you want like some astroid fields and whatnot.