Just enough so that the beauty category can't go any higher.
But remember you have a limit of 12 poffins, my recommendation is, do not feed it a poffin with a less than than a level 10, and also if can, first bake the 12 poffins and when you finish, start by feeding the ones with a higher level first. Otherwise you will fail at it and will have to breed a new feebas.
Try Using the fallowing natures: Modest, Mild, Quite, and Rash. Those are the ones that love dry poffins.
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You do not have to give Feebas a Water Stone to evolve it. To evolve Feebas you need to trade it while it is holding a Prism Scale.
First, there aren't any poke blocks in Pokemon Diamond. They use poffins. Poffins and poke blocks are like the same thing but poffins are bigger. Now to evolve Feebas you need to give it Dry Poffins. Dry Pofffins are like blue poke blocks. Make sure you feed your Feebas HIGH LEVELED DRY POFFINS so it will be easier to win contests (only beauty contests). Feed it those types till your Feebas won't eat anymore. Then train your Feebas about 1-3 levels and it will evolve into Milotic. -To get Dry Poffins you have to bake berries that are dry. Go to Hearthome City and into the house beside the Poke Mart. You will be able to cook poffins there. -To cook Poffins you NEED a Poffin Case. Go into the Fan Clube in Hearthome City and talk to the girl and she will give you a Poffin Case. To get a Feebas go to Mt.Coronet and surf on any body of water you find. You will need to fish using a Super Rod. It will take lots of time because they are rare, or just trade one or miggrate from the GBA games into your Diamond. Note: Feebas can only be found after you get the National Dex. To get one you need to see all 150 Pokemon in Sinnoh and beat the Pokemon Leuge.
Give it a LOT o' haircuts then level it up.
Make lots of Beauty poffins out of berries and feed them to your Feebas. Once its maxed out, your Feebas will evolve.EDIT:Get a female feebas, I can trade you one for almost nothing, I have plenty ;), breed it until you get a nature that likes dry pokeblocks/poffins, and feed it dry ones. There is a good nature chart here:http://www.psypokes.com/lab/natures.phpOR you can PM me (Starcruiser322) and I'll give you a milotic, again for almost nothing. I have Wi-Fi.
catch a feebas and give it beauty poffins until its beauty is full then get it up levels hope that helps