Well, first, you need to find a sweet berry. What you do, is check the berry's "tag." If the sweet level is high, then you'll most likely get a sweet poffin. If it is low or null, you most likely will get another kind of poffin. Here is a link to the different kinds of berries and how to find them. Click on the berry to see what contest stats it raises.
http://bulbapedia.bulbagarden.net/wiki/Berry
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Unfortunatley, there is no way to make a Mild poffin except for the one that the guy in the contest hall gives you when you first receive your poffin case. However, there is sililiar poffin called the Sharp poffin that is similiar to the mild poffin and is the same colour. It is often high-leveled. In order to get a Sharp poffin, you must have different berries (ex; Mico berry, Starf berry) and 4 other players.
A Wiki Berry.
The chances of making a Lv. 99 Poffin is very rare. Unless you cheat with an action replay or hack it.
The Poffin maker's house, it is left from the Pokemart in Hearthome City and ask the lady in front of the Poffin Maker. If you need a poffin case go to the Pokemon Fan Club and talk to the old man at the couch.
The type of Poffin you make in Pokemon Diamond, Pearl and Platinum is dependent on the type of berry used to make it. In the case of the Dry Poffin you can use Chesto, Oran, Wiki, Pamtre, Charti and Micle berries.