First, you must decide the outside dimensions you want for your mobile home design. Most companies that build mobile homes use a "straight-line" design for their construction. They are 14' by 32/36/48/60' or 28' by the same lengths, and in the case of triple wides, you can go up to 36' by the same lengths. There are some other designs you can utilize, and they cost accordingly, so decide just how much you're willing to pay for.
Now decide what basics you want: how many bedrooms, bathrooms, extra rooms, how big of a kitchen, dining room, family rooms, etc. Using the dimensions above, decide how you want to place these rooms. Usually, the best placement is to put the bedrooms in a split plan. The master bedroom will go at one end with the master bath and the other bedrooms and baths at the other. The other rooms, such as kitchen, pantry, living areas, will go in the middle of the setup. If you've chosen a non-traditional design, the room placement will vary. Keep in mind that you have to work within the limits you've placed on yourself by the outside design.
Now for the extras. Decide where in each bedroom you want the closet, any built-ins in whatever room, where a linen closet should be, shower or tub size and placement (left or right hand entry), electrical circuits, plumbing, venting and heating/cooling systems. This is where you can get really creative, but remember to keep it simple and straightforward.
Once you've committed all this to paper, you can take it to your local mobile home dealer and present it to him. If he's reputable, he'll contact several manufacturers on your behalf and find one that will be able to build your design for a fair price. Be forewarned, however, that your price will vary according to the number of square feet in your design.