Raising a Mage differs a lot from other Professions in that you only recieve XP for a success and not for a failure. While originally much easier to raise, since 1.28.0 the system has been completely changed and requires much more time and investment.
Raising a Mage requires a lot of Smithmagic Runes so you should either have a large stockpile beforehand or be willing to buy more as you go through them.
The basic principle of the maging system is that to gain optimal XP for your Mage the item being maged should be twice the Magus level. Higher power runes yield more XP, but they are not cost-efficient so it's usually better to use basic runes.
It is not usually cost-efficient to craft or buy items twice your magus level, but using items too far from that yields suboptimal XP per rune.
As well as swapping out items you should also aim to get items with multiple stats, and bring a stock of the relevant runes. When maging the item you should aim to mage one stat close to, if not, max, and then mage back the other stat. This is usually quicker (although requires more effort) than simply sticking a large amount of the one rune on and leaving it to run. Using the "custom gauge" interface near the health panel can help a magus keep an eye on how close they are to the next level.
You can also speed up the first 10 or so levels by using Experience scrolls. This can actually be more efficent than runes as, depending on the Mage you're raising, it can take a lot of runes simply to get from 1 to 10.