Each spell is treated as a skill - you build up skill in the spell and roll to get a success. This burns fatigue.

Like combat in away - build the ability with Development Points and roll. Magic is often a pain, ingredients for an authentic "feel", fatigue burns away fast unless you have spent a LOT of DP in just one spell/skill.

I favour the "vancian" take - use the "hang" spell as a mechanism for loading up spells. Character takes a day or more memorizing spells as a "ready list" up to the fatigue capacity - treat as ammunition - fired with no fatigue cost but limited to the list of memorized spells.

Basic set Characters covers it - inexpensive e-book available from Steve Jackson Games - I have the Magic book which is much better but you need to be selective about what you allow. GURPS Thaumatology is a very good source book - but be VERY selective about what is used. Each player should design their own set of laws in discussion with the GM and other players.

So, treat a spell as a skill for success purposes, burn fatigue (see the book for the cost) depending on skill, get the effect. Fails are a waste of fatigue, critical fails use the table - I find it a little predictable with the GURPS 3d6 dice. I'm converting combat to the Rolemaster charts and Magic will follow - more fun, more splat, more frustration for the players.

Spending today reviewing the Alchemy rules - a task for minions if you can find a clever minion.