There used to be a fair few tutorials dealing with spells, etc... However iirc all were geared for the old utility IEEP.
DLTCEP is much the same in use as IEEP was, it's just capable of "more". Of course this makes it resultingly more confusing. For starting all modding I'd make a few suggestions:
1) Make a backup copy of your Override Folder, Dialog.tlk, and chitin.key files. If all goes to crap, you can fix your game without reinstalling with these files.
2) Check out
IESDP. All that is known about the
IE engine is there. For your spell question, it's handy to be able to scroll through the
BG2 Effects list. It'll let you know what you can do with spells/items and what parameters to set.
3) Start by opening and modifying existing files. For lower resistance, open that spell. Check out the extention header where all the action takes place. Modify it as you wish and save it under a new spell name. (Or save it under a new name before doing anything, just in case.) If you need to, open another spell in game that has an area of effect to mimic it's settings. Something like the "Sleep" spell for example. Note the targeting differences and try to copy it.
4) Test it in-game. If it works, export a copy of the spell to a folder somewhere. (Use the .tbg export to keep the text attached to the spells.) That way if you do have to start from scratch due to a corrupted game, you don't lose all your work.
Good luck, and feel free to pop in with some specific questions. Some of us are old grognards as well.