Are they simply single lined schematic designs, or are you trying to represent isometric details showing a good amount of detail?
To at least get you started, try this. Type SNAPSTYL at the command line and set it to 1. This gives you the "Isometric Crosshair". Use the F5 key to rotate the crosshair in the three different orientations it has. Next, be sure to set your Polar Tracking on, and set it at an incrementing angle of 30 degrees. As you draw, just keep in mind the isometric perception and you'll do fine. When it comes to circles, you can use the ELLIPSE command and choose the "Isocircle" option. This gives you a view of a circle at an isometric angle very easily.
This is the very basic setup. Come back with any more questions you may have.