How do you enchant an item to reduce magicka cost by 25% for a specific school of magic?

Also is this build appropriate for a warrior/spell caster?

http://skyrimcalculator.com/#36301

That build will be complete by level 49 so my remaining perks can be invested in the warrior skills.

You just need to know the appropriate enchantment (Disenchant a suitable magic item). if you have max enchanting + rank 5 enchanter + insightful enchanter + grand soul gem then you will have -25% spell cost for that enchantment.

Incidentlly that build looks terribad for a warrior-anything, how exactly do you plan on using impact if you are using a melee weapon? ;p - I'd suggest focusing one just one or two elements instead of trying master all 3, it's not like you can make triple enchanted weapons so having all 3 elemental enchanting perks is a waste right off the bat.

I'll assume you know how enchanting works. Otherwise, you can look up the Enchanting explanation on UESP. Just simply use a Fortify (fill in magick school here) enchantment. These enchantments are limited to items that are worn (armor, clothes, boots, rings, amulets, circlets, etc.) So no "Swords of Easier Destruction".

So I don't need alchemy at all to achieve 25%?!

Correct. using alchemy you can achieve closer to 29% (Personally i use x3 29% enchantments for 87% cost reduction, which is trivial and saves a couple of enchantment slots)

The fortify school group of enchantmets can go on head, neck, chest, and finger items. The weaker fortify school + mana regen goes on chest slots.

Simple enchanting though is:

Find an item with the enchantment you want, take it to an enchanting table, then disenchant it. That will let you "learn" the enchantment.

Than at the enchaning table Select the item to be enchanted, the enchantment, and the filled soul gem. Click the craft button.

I'm not sure who high your enchants go without adding alchemy to the equation.

Uggh. I wouldn't even want to max out alchemy anyways just for 29%, even with well rested and thief sign, a potion gives me something like 1/15th of a level up at level 15 alchemy!

Thank you

I have made this build which I think is better now

http://skyrimcalculator.com/#36322

Yeah, not much point going to those length unless you're a massive min-max'r, or have a build that incorporates both skills anyway (http://skyrimcalculator.com/#36320)