I''m just starting to figure this out myself so you may have already used these techniques but I found a few things seem to help me so far. I start with a square and draw the things I would be able to see from a top down view such as the top of a wall, roof of a building or a road then I rotate the tile 45 degrees and reduce the height by 50%(or less depending on the angle you want) then I add the detail that would show up from an angle such as the side of a wall. I''m still working on this technique so I may change my approach later but it has helped me to at least make something that looks like its angled the correct way.
It all depends on how you want the ISO tiles to look. You can just create a square tile on a layer seperate from the background color. Then goto Edit/Transform/Rotate, and rotate the square tile until it''s a diamond shape. If you want to give the tile depth you''d use the Edit/Transform/Perspective option.
I suggest making a template tile, one that has just a black outline of the size of your tile. Make your tile as a large square, making sure it''s seamless or whatever you want. Copy and paste the template tile onto a large new document. Then copy and paste the large square picture into a new layer. Select the new layer, choose the transformer distort. Drag the corners of the large picture to match the corners of the smaller template tile. Apply the distort. Select the template tile layer using the magic wand select all the space outside of the template tile, select the large picture layer and clear the selected area, getting rid of all the overlap from the distortion. Add other details and such. This is the method I used to use when making isometric tiles, you''ll probably have to mess around with it a few times so that you can distort it correctly without losing to much off the sides.