So let's say you're making a spell to create and control a natural phenomena. Since the creation of it will always be primary, it's a Creo spell. But adding control is a big deal, so that's a Rego requisite which will add +1 Magnitude because it's a "additional effect". But let's assume that it's part of the level 15 Wonders guideline and that lets us ignore that additional magnitude, but remember that it's still a requisite for inventing and casting.
Now the spell could be of any natural phenomena at random, so CHOOSE ONE elemental Form as your base - for this example we'll choose Auram. Now add requisites for Terram, Aquam and Ignem. These don't add magnitudes, as they don't really add to the spell other than to make the effect a random element, which is seldom helpful, and the spell will ALWAYS be resisted by Auram Resistance because it's an Auram spell at it's heart (this is why you might choose a specific Form over others, perhaps your Penetration specialization is in Auram).
When you cast the spell, apply all requisites as normal, so you'd cast with the lowest of your Cr/Re, and the lowest of your Aq/Au/Ig/Te. The exact same thing applies when inventing it in the lab - use the lowest applicable technique and form, but if your lab (or talisman) has (in this example), a bonus to Creo or Auram, they would apply as it's a CrAu at its heart.