The basic starting of a quake 2 mod has 2 methods...
1 : create a desktop shortcut of the quake 2 executable and in the properties for that shortcut just enter "-game giex" in the area labelled "Target". Make sure you enter it after the quotation marks that enclose the game executable's directory.
2 : Start quake 2, open the command console with ~, type "game giex" into the console, type "coop 1" into the console, finally type "map base1" to get started on the 1st singleplayer map.
After the map loads, there's a light setup for character creation.
First of all, your actual player name is your character name. The character file is saved under this name. Ignore the "login" menu for a second and make sure you enter "name nameyouwant" into the command console to change your name as desired. Once that's done, use [ and ] by default to navigate to "create character" in the 1st in-game menu. Press enter and then it asks you for a password to protect this character under this name. Enter whatever password you want, you'll have to enter it again every time you play under the same name; each playername is saved with its own password.
After logging in, you get to choose your character class.
Soldier is good offense, Technician is good defense, Cleric can heal themselves well, mage gets great bonuses to spells and magic weapons, and the vampire starts with some life-leeching abilities.
Check in the mod folder. Open the file "autoexec.cfg" in there to see the changes and additions to controls.
By default, f9 shows your character status, f10 shows what's equipped, f11 shows your spell list, and f12 shows items you've picked up. Also by default, W key will identify an item that your character is touching and Q key will pick the item up. With those commands you should be fine knowing the rest of the quake 2 default controls. Just remember that your inventory (opened with tab) will contain very new and different items in this mod and that most of them take a second to equip. You won't be fast-swapping weapons in this mod.
One last thing, spells. bind keys in the console (bind ) to the following commands... "nextspell", "prevspell", and "+castspell". That takes care of spellcasting. The reason you can't cast some spells right away is because they require you to level them up by taking other actions such as attacking enemies. Then they will unlock as long as they are equipped in your memory.
That should be enough to get you started! Good luck leveling!