There are many simple ways:
-Get yourself a program named TrueSpace or Milkshape, which can export Models into this format (and create them)
-Get a Plug in for any other 3D modeler (Maya Plugin and others are included with the DirectX9 SDK)
-Create 3DS Files using almost Any Modeler (3DS is more or less a standard format) and use the Program supplied with the SDK to convert it into .X format
-Open the File in WordPad and u can create/modify them, since they are not stored in any compressed/unreadable format (altough for this u need to know how the file is build)