The difference between a 'burn phase' and 'conservation phase' is essentially the resetting of stacks. A burn phase is always ended with an Evocation (at 35% mana). The best way to initiate a burn phase is to AB until you're around 85% mana, use flame orb... then pop mana gem, arcane power, and any on use trinkets you might be using. In addition you should be popping mirror images after AP is popped (it shouldn't cost a GCD if glyphed). After that simply spam AB until you reach 35%, and Evocate.
During a conservation phase, you should be resetting your arcane blast stacks in order to stay around 85-90% mana until you can begin another burn phase. Depending on your gear level, you can cast 3-6 (sometimes more at higher gear levels and while using Shard of Woe) Arcane blasts before clearing your stacks with Arcane Missiles. You really have to manage your mana well in this phase. You don't want to dip too low or else your next burn won't be as powerful. Also note that the only time you should be clearing your stacks with an ABr is during movement.
So the whole conservation phase you should be doing Arcane blast > clear stacks with Arcane Missiles. You have to kind of 'eyeball' your mana usage in this phase. Mage armor should give you decent regen but not enough to allow for constant AB spamming, especially at low gear levels. You're going to want to begin your burn phase when your Evocation is at a point in the cooldown where it will be ready once you are finished with the burn. This is usually when there is 20-30 seconds lefts on your Evo CD (once again depending on your gear and the presence of Shard of Woe).
Arcane is one of the specs that can really vary depending on the mechanics of fights as well as the gear that you have. It honestly becomes much more forgiving at higher gear levels, but it's good to start out at the bottom so you learn the repercussions of bad mana usage.