Okay so, I've got a few fortify smithing potions and a few fortify enchanting potions and I have everything I need for my daedric dagger.

Do I drink a smithing potion, make the dagger.
Drink another, upgrade the dagger to Legendary.
Drink an enchanting potion and add my enchantment?

I'm not bothering with Armour and jewellery and things that will enhance it.

Also I'm planning on using Mehrunes Razor and a Daedric dagger with the Banish enchantment.
Should I use a different enchantment?

Or should I use 2 Daedric daggers, one with banish and one with something else?

just drink the smithing potion when upgrading the dagger. You always make the same dagger regardless of skill. And yes, drink the other potion before you put the enchant on, and be quick, you only have 30 seconds and time keeps counting while crafting. Also, be careful with the bonuses, if you start stacking fort damage enchants and potions it will be funny for 10 minutes and then get really boring really fast.

Okay, thanks!