Spellcrafting, or spellmaking, is the art of creating unique spells, usually through combining multiple existing spells. A custom spell can have more than one effect (such as frost and fire damage). The magnitude of the effect is proportional to the magicka required to cast the spell. Multiple effects have compounding magicka requirements.
By game
Usage
Spellmakers
Spellmakers of the Mages Guild provide services of spellmaking.
Altar of Spellmaking
Spellmaking can be performed with an Altar of Spellmaking.
Effects
Effects are only available if you already have a spell including that effect and the ability to cast that spell (both sufficient rank and sufficient magicka). (Scrolls do not count).
Choosing spell properties
You can also set area damage and duration to a spell when selecting the damage. Along with that you may choose the range of the spell as either on touch, on target, or a spell on yourself (can determine amount of magicka needed). You may even mix and match spell types and schools, such as making a single spell that casts Charm on touch and Fortify Personality on self.
In general, Destruction spells that have a longer duration for the same final damage amount will cost less magicka to cast than one that deals the entire amount of damage over a single second. For example, a fire damage spell that does 50 damage over 1 second should cost more to cast than a spell that does 10 damage over 5 seconds. It is also possible to make a drain health spell that drains 100 damage that lasts for 1 second, meaning that anything with less than 101 hp will die instantly. The spell is cheap, though won't necessarily kill every thing, and is a very strong finishing blow on the weak.
Some skills, such as Armorer, Speechcraft, and Mercantile, may be exploited by creating a spell that fortifies one's proficiency in that skill by 100 points for 1 second. Simply cast the spell and call on the menu related to that skill (repair menu for Armorer, barter menu for Mercantile) and enjoy the boosted stat. Such spells usually cost less than 20 magicka and may be cast by almost anyone. Charm can be exploited in the same way.
Up to eight fortifications (racial boosts, enchanted items employed, active spells) can be stacked. To stack spells, you must create different spells with the same fortification. Since spells take about 2 seconds to cast, stage the durations to be 2 seconds shorter for each successive spell in a sequence. Name the spells in alphabetical sequence, so after the 2nd spell the 3rd and subsequent spells will kindly be positioned under the cursor when tab is pressed to bring up your spells.