The best way to do isometric like the ones above is pixel by pixel, I reccommend a painting program with a grid option for this.. (I use Multimedia Fusion and Game Maker's built in editors) from there you normally start with a base (usually an isometric "Square"), to do this just make a diamond shape like this(though it would normally be bigger): 
         ..
        .  .
       .    .
      .      .
      .      .
       .    .
        .  .
         ..
And stretch it horizontally to double width... 
            ....
          ..    ..
        ..        ..
      ..            ..
      ..            ..
        ..        ..
          ..    ..
            ....
You can do the same with a circle, simply stretch it to double width.. Now to make a 3D looking shape.. a cube.. Double your diamond and move the copy up to the height you want your square... 
            ....
          ..    ..
        ..  ....  ..
      ..  ..    ..  ..
        ..        ..
      ..  ..    ..  ..
      ..    ....    ..
        ..        ..
          ..    ..
            ....
Add edges and remove lines that shouldn't be seen.. 
            ....
          ..    ..
        ..        ..
      ..            ..
      ....        ....
      ..  ..    ..  ..
      ..    ....    ..
        ..   ..   ..
          .. .. ..
            ....
Then just fill the sides or shade them if you wish, you can do the same for most any object, just remember to keep it twice as wide as it is high :)