Quality Guide (seriously):
Platinum Nugget (4x platinum leaf, 2x vitriol)
Fail over and over (and over and over and over) until you hit synergy 70.
1- Claim furnace, trade materials, start synergy
2- Spam add whatever fewell you like.
3- Thwack when it's about to explode (free skill up opportunity without expending fewell)
4- Fail synth, take materials out
5- Rinse and repeat.
Never end the synergy, you don't want to make the nugget, you just want to fail over and over. If you want to save money, buy on AH (or craft) your own fewell instead of buying from NPC -- NPC is way overpriced. Don't lower pressure or pull safety lever, it's just a waste of fewell while leveling. If you want to save a little bit of time, sub WHM so you can clear status effects after an explosion. They only last 10 seconds anyway, but you can't interact with the furnace with any debuff on you.
Every five levels, you can use the synergy NPCs to level up a particular skill. In my opinion, the most useful ones while doing a real synth (not a fail-synth) are fewell feeding (increase likelihood of large element increase or perfect alignment to target), safety lever (decrease impurity more per pull), and pressure handle (decrease pressure more per pull).
It's very (very very very) random, but you should be getting something like 2-5 skill levels per hour (more if you're very lucky, less if you aren't). You should be using something like 750 fewell per hour. It will take A LOT of fewell (and time) to get to 70, but that's the only expense.
Some people say that you get a better skill up rate by starting with something closer to your level, but I call BS on that -- I started with Gold Nugget and went all the way to 60 on that (when the cap was originally 60, gold nugget took you all the way there), and my skill up rate was on par with everyone else's. If you want to get 80, you'll need to find some recipe in there that you can do, as all of the skill-up recipes post-70 require some high level craft. As far as I know the only ones that can get you all the way to 80 are the veteran crafting stalls and the new pieces of equipment with Voidwatch materials.