Creating any spell, whether Formulaic or Spontaneous really isnt that hard, even if the whole scheme seems convoluted by pages upon pages of text at the start.
Think of it all like a big box of legos, each piece one of 15 different colours.
Now when sitting down with your troupe. just explain that it all boils down to this basically (there's always more complexity to add later of course):
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Decide what effect you want your spell to have
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Look at the Techniques and decide which Technique (Cr, In, Mu, Pe, Re) best serves the action being attempted
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Look at the Forms and decide which Form (An, Aq, Au, Co, He...) best serves objective.
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Now, if spontaneous, tell the player(s) to look at the score they have assigned to the chosen Technique and the chosen Form. Add those two numbers...
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Now tell players to add their stamina score
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Finally decide as SG whether the area in which the spell is being cast is subject to any Aura and what the level of that Aura is. If casting in a church or middle of a city you will likely being assigning a Dominion Aura of no less than 3 and maybe as high as 4 or 5 which would make all but the lowest level spont effects possible for starting characters.
If the above then the equation would be: Tech (score) + Form (score) + Stamina (score) - 3*Aura/2 for spont spell with fatigue expenditure.
If the Aura is magical then have them add the Aura level to casting total for a resulting: Tech (score) + Form (score) + Stamina (score) + Aura level/2. Add a stress die roll to the equation (before dividing by 2 of course) and presto either a successful casting of spont magic or a fizzle or a botch. We hope for no botches for your people on their first night out
By way of example:
Ardath of the Ad Fons PBP scenario in the PB section of the forums is being chased by a group of werewolves and wants to elude them long enough to put sufficient distance between them and herself so she can choose another route back to the safety of the covenant. It is night and there are no stars so the sky is a pitch black.
Being particularly adept in both Rego and Terram magics with particular focus in obsidian, she decides the quickest evasion is straight up.
So she first needs to create the disk on the run. She spontaneously CrTe an obsidian disk upon which she can raise herself into the air...
Her Creo score is (serf's parma) say 6 and her Terram score is, say, 8. Her stamina is 2 and we shall say that since she is exiting the mystical forest near to the covenant she benefits from a magical aura bonus of 2 (higher the deeper in you go)...
So: Creo (6) + Terram ( 8 ) + stamina (2) + Aura (2) + Creo (6) for the minor focus in obsidian +stress die/2.
Looking at the Creo Terram guideline box in the core rules on page 153 we see that base effect for "creating stone" is level 3. The base size for rock in Terram magic is 1 cubic pace or (according to most here) 1 cubic meter/yard). By mentally designing the configuration of that volume of rock into a a sturdy 6 inch thick disk she should have plenty of material for a nice sized disk in overall circumference to bear easily standing or sitting.
So she figures: base 4 (1 more than listed guideline since it is stone not dirt being created), +1 touch, +1 diameter (20 battle rounds or 2 minutes), Target: Individual (no cost) for final effect level of 10.
Now she rolls the die and gets an 8 for a casting total of 16 (32/2). More than enough to create the disk she wants and sufficient to change one of the spell parameters (Range, Duration, Target) by one step if she so wishes since this is spont magic (see post roll casting adjustment rule on page 82 carry over paragraph from previous page under Conc Table box).
She is content to just have the disk made so doesnt change any parameters for this effect. Next round she sponts a Rego Terram effect to lift the disk and thus herself into the air.
Her Rego score (serf's parma) is say 8 and her Terram score is, say, 8.
All other casting additions remaining the same as above and again benefiting from her minor focus in obsidian (since she is again effecting an obsidian object) her spont casting total is:
Rego ( 8 ) +Terram ( 8 ) +stamina (2) +Aura (2) + (either Re or Te score once again since both are equal) = 28 +stress die/2
Looking at the Rego Terram guideline box on page 155 of the core rules, we see the base for controlling dirt in a very unnatural fashion is base 3. Since, again, she is controlling stone not dirt its one magnitude higher or base 4.
so base 4, +1 touch, +1 concentration, Target: Individual (no cost). Up to the SG if he wants to insist that anything other than movement straight up to a hovering position of any height (i.e. ability to will the disk to move horizontally in the air to take her safely home) would require an additional magnitude of complexity, but for simplicity sake let's say no at this time and come up with a spontaneous effect level of 10.
On anything other than a 0 (botch) she succeeds without problem and levitates the disk and herself up and away through the night sky unseen by the prowling werewolves on the ground below (thanks to the pitch black colour of the obsidian disk against the dark night sky.).
And that's an encyclopedic demonstration of how spontaneous magic is calculated and performed.
Cuchul will be proud of me (for verbosity anyways)!