As a "General Guideline", I recommend finding an existing item that is similar to what you want, and then making minor substitutions and modifications, so that the end result is effectively equivalent in balance to official content.
Beyond that, the method for properly balancing an item depends entirely upon the item's effect. Items that are nearly always useful, such as Additional Modes of Movement should almost always require attunement, whereas items that only have niche value shouldn't require attunement unless they are particularly powerful.
Basically, design your ideal item, and then ask yourself "Given these benefits/limitations how likely am I to choose this item over another item of the same rarity?" If the answer is greater than 50%, then it should be scaled back. Magic items should be exciting, but not so much so that they are the obvious choice.
General question for everyone: What are your personal guidelines/limits when you balance an item? For example:
1) Attunement vs Unattuned?
2) Charges vs."always under the effect of" vs. "Cast without components?
3) X/day vs [option from #2]
most of these are highly situnational but 2 and 3 constant effects are really strong
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As a "General Guideline", I recommend finding an existing item that is similar to what you want, and then making minor substitutions and modifications, so that the end result is effectively equivalent in balance to official content.
Beyond that, the method for properly balancing an item depends entirely upon the item's effect. Items that are nearly always useful, such as Additional Modes of Movement should almost always require attunement, whereas items that only have niche value shouldn't require attunement unless they are particularly powerful.
Basically, design your ideal item, and then ask yourself "Given these benefits/limitations how likely am I to choose this item over another item of the same rarity?" If the answer is greater than 50%, then it should be scaled back. Magic items should be exciting, but not so much so that they are the obvious choice.