The Basic's of Isometric Map Drawing.

getting the Isometric isn't as hard as you'd first think, 1st draw a grid on a piece of paper, turn it 45 Degrees and tilt it 45 Degrees what you see is an Isometric Grid.

Now the 1st point of your grid is farthest away form you and if you number you're grid
will be 0,0 1,0 is 1/2 tile length to the right and 1/2 tile height down 2,0 is 1 to the right and 1 tile height down, etc...  

so to work out an x,y position of a tile on screen from you're grid you:-  

TileXpos=((x-y)* TileLen/2)+ScreenWidth/2
TileYpos=((x+y)* TileHeight/2)

if your tiles are 12x6 your tile 0,0 is placed at screen coords

TileXpos=((0-0)*12/2)+640/2 = 320
TileYpos=((0+0)*6/2) = 0   etc.

  you really need a bigger tile that 12x6 I use 74x37 you can speed this calc up by pre calculating some of the values eg.

12/2  = 6, 6/2 = 3, 640/2 = 320 so
TileXpos=((x-y)*HalfTileLen)+HalfScreen
TileYpos=((x+y)*HalfTileHeight

so long as you draw your tiles from left to right, from back to front you'll end up with an Isometric Map.
so :-

For y=0 To 9
    For x=0 To 9 '(a 10x10 map)
        TileXpos=((x-y)*HalfTileLen)+HalfScreen
        TileYpos=((x+y)*HalfTileHeight
          DrawSprite TileXpos,TileYpos,Map(x,y)  ' where DrawSprite is your own DX sprite Blit code
      Next x
Next y

This should give you some ideas as to how to go about getting an Isometric map, as you can see the principle isn't that difficult, you can speed the whole thing up again by realizing how the sprites are drawn. IE. the next tile is drawn half way across and down from the previous one. so you could just add these values instead of calculating them each tile, but I'll leave that up to you.

Good luck with you're programmes.
Rob.