If you were the caster and you have spell remnants (the thread that was knotted, the wax of the candle that was burned, etc.):
Bring the spell remnant back into Magical Space (whatever space, mindset, or rituals you require in order for your mundane actions to become magical).
Perform Undoing actions on the spell remnants. Cut or untie a knot, melt or pulverize wax, mix ashes with magically nullifying substances such as salt and iron, etc.
While performing Undoing actions, also give clear linguistic Instructions on what you expect to happen. These instructions may be spoken, thought, signed, written, read, or be imposed upon your Magical Space in any way you prefer.
Tie your instructions to your physical actions, such as:
- This spell is undone as this cord is cut, the magic binds no longer
- This spell is melted and reduced as I melt and reduce this wax; the magic dwindles into nothing
- Ashes to ashes, dust to dust; my spell is consumed by this iron rust - the magic no longer functions
I highly recommend using clear and direct sentences to the effect of "I am undoing this spell and I don't want the magic to work any more" (for those who only use present-tense intents: "this spell is undone, the magic has stopped working."
Dispose of the remnants in a permanent way.
If you were the caster and you do not have spell remnants:
Consecrate a new object (new thread to be knotted, new candle, new piece of paper) to represent the old spell. Ideally, this object will be chosen or modified to resemble the vibes of the old spell as closely as possible.
Any consecration ritual will do. The goal is to give a new physical body to your old spell; you are making an "poppet" of the spell.
Once the new physical object has become synonymous with the old spell, perform the Undoing and Instruction method that was described above.
(New candles should probably be snapped in half or burned upside down; burning them normally is more likely to empower the old spell).
If you were not the caster:
The "poppet" method above will tend to work if you know what you're doing with things. However, one of these might be more reliable:
Method One: Bless a thread with binding, limiting energies (Saturn is ideal) and trap the unwanted spell by tying it in a knot. Then burn the thread, ideally while Instructing what is supposed to happen.
Method Two: Carve the end of a taper candle so that the wick is visible on the bottom. Treat this bottom as your new "active" end, and draw correspondences from the new active end, downwards.
Coat the candle with a pepper-infused oil, score the surface lengthwise with iron nails, or otherwise mark the candle with destructive and banishing correspondences.
While doing this, Instruct the candle that its job is to banish, burn away, and destroy the unwanted spell. Light the candle. (A temporary holder may be obtained for the awkward shape by filling a deep dish with sand, soil, or salt).
Notes and Bits (Beyond 101):
It is generally wise to construct all spells with "kill codes" in case you need to undo them. This is essentially a special sort of intent that you build into the spell which undoes the spell on your command.
To avoid intrusive thought anxiety, it is almost always best to have this "code" require a specific set of physical actions, such as writing "Spell Cast on [date], be undone and begone" seven times over on a piece of paper.
Undoing a spell does not teleport you back in time to before it was ever cast. Imagine a valley with a river. A spell is cast that dams up the river. The entire ecosystem of the valley changes because the river was stopped.
Later on we can dismantle the dam and let the water flow again. But the valley will not "magically" go back to what it used to be. The spell may be undone but there will also still be significant changes already affected on this valley.
Record keeping is one of your best allies in knowing how to undo your own spells. The more you know about your spell (or anyone else's), the easier it is for you to undo or counter.
Once a spell is undone, you may still have more work to do. Suppose someone hexes me. Even if I undo the hex, my house may still be filled with nasty energy and I can still have anxiety. I will have to do a good self-cleansing and home-clearing before the effects are fully erased. Undoing a spell is not the same thing as cleaning up after it.
In sticky situations, you may also want to cast additional spells to protect against the effects of the unwanted spell.