1) First Name That Spell!
If you want to cast a spell you had better have a snappy name for it, suitable to the Discworld. Discworld spells go (mostly) [Inventor's Name] [Adjective] [Effect]. And a bit of alliteration always helps.
Example: The sergeant wants you to cast a spell to see if someone is telling the truth. Thinking you had better do something to impress her you say "Yes, I think this is the perfect occasion for Verdain's Verdant Veridicator!"
2) Tell Me What You Want to Make the Spell Do
Describe the effect. Don't be too ambitious or you'll get into trouble at the next stage.
Example: You turn to me, your smiling GM and say: "I'd like the spell to make someone glow green if they tell a lie under its influence."
3) I Decide What Level the Spell Is
There are eight (of course) levels of spells from lighting cigars with flames from your fingertips (Level One) to painting the sky pink (Level Eight) and the GM will decide what level is appropriate to your spell. To say that the GM can be influenced by being offered bribes of food or somehow getting Charles out here is merely to state the obvious.
If the spell is of a level higher than your current wizard level (= number of dice in the Wizard trait) then you suffer a penalty die for every spell level above your current Wizard level.
4) You Roll the Dice in Total Ignorance Of . . .
. . . what you need to get. Which the GM will have made up when he assigned the level. If you are casting a spell on someone else then they will resist with some appropriate trait or with 2 dice in any case. ROLL: 3 dice + the relevent # of dice for the spell (see above)
In the case of the Verdant Veridicator, you would definitely have to overcome the target's resistance if they didn't want to tell you the truth.
Any attempt to use the same spell in the same day is at a penalty die.
5) You Cross Off One of Your Magic Shots for the Day
You can refresh your magic pool by: Waiting. It will return to full each dawn. Meditating. This can involve either Yoga or for U.U. grads eating a Big Meal. Tantric Sex Practices also work. But as a U.U. grad it is far safer to stick to Big Meals.