Firstly with WHM because it was way harder (in lvling up) than BLM.
From lvl 1 - 10 its a given and im pretty sure most ppl will agree, that it will be gruelingly long. And yes you will die (unless your getting PL) the only thing i can say is use dia at the beginning of every fight nomatter what. ALWAYS use protect even if it doesnt seem to help, because it does.
From lvl 10 - 20 You should be in Valkurm dunes cause thats where most ppl go. Aquaviel is a great spell and you should get it along with blink, and shell. The bar spells are good to have as well. My only advice (since its much easier to pt in the dunes than solo) is you DO NOT have to keep your party members health full to the max. Its a waste of MP. Just try to keep it in the yellow or close to it. Everything else is self explainatory. Oh and I know you learn cure II, but dont just start spamming it. Keep using cure. CureII takes more mp and you get weaker rusults than if you had used cure twice. Dont use Diaga cause its pretty useless.
From lvl 20 - 25 Quifim Island is your best bet. Use sneak to travel past the orc's and ghouls at night. Yes they aggro on sound not sight. Sneak will also get past the Evil weapons. As for spells, the same as above, Raise and invisible is great but dont use them in battle, its really pointless and burns mp. Always cast regen on your tanks and NOT the mages (no duh right)
Lvl 25 - 32 Kazham the EASIEST way to get exp points and fast. Now your loaded with protect II, Protectra II, Banish II and Stoneskin <3 Of course use Stoneskin and Blink because they do stack. Go Yutunga from 25-27 and then Yhoater Jungles from 27 - 32. Also Banish II is a much more powerful spell, but it burn MP like a Bad rash so be warned.
I cant give you more infor for WHM because I have not gone past lvl 32 whm. The same with BLM. So here we go
Lvl 1 - 9 (blm) The same as whm. Long and grueling execpt you dont get the chance to heal yourself. Nukes are pretty weak, but worry not they will get better. Like whm your just nuking and meleeing. Always cast poison first and then nuke away.
Lvl 10 - 20 Finally you get fire, Bio, sleep and they all will be useless if not very little helpful. But keep at it. use Elemental seal as much as possible and youll be doing enough damage to make the tanks envy you. Remember get your INT as high as possible is seriously helps.
20 - 25 not bad not bad. Quifim Island once again and it gets faster. MB will be crucial here but you should be used to the timing of your spells by now. You learn more elemental spells and you should have pies, yag drinks etc. There really aint much to say, but electricity should be your goal here. And yes you learn thunder here i believe

25 - 32 Kazham again yay! Finnaly if you get your Int to AT LEAST + 12 when you hit lvl 29 and you have seer's gear the tanks will be envious of your damage dealing. Firaga and Aeroga are great spells here but like all GA, dont used then unless you Know and see the coast is clear. Its best to not use them at all unless you like Deathga and so does your party. Youll be fighting gobs and madrogora's so use fire to its fullest, aero is great and i use thunder just to be mean. Tractor and escape are great spells, but you really cant use them here. Apart from that ive done 169 dmg with thunder in Kazham and that my friends aint no lie

lvl whm and blm beyond
I know my tips really arent THAT helpful, but at least it will give noobs who want to become a white or black mage an idea. Well thats all i got. Hope it proves some help.