Spellhearts are permanent items that work similarly to Talismans. You affix a spellheart using the Affix a Spellheart activity, which is otherwise identical to Affix a Talisman. The limit of one talisman per item remains—an item can have one spellheart or one talisman, not both. When casting a cantrip from a spellheart, you can use your own spell attack roll or spell DC if it's higher.
Unlike with activating Talismans, you can cast spells using Spellhearts affixed to other peoples' items, but you must be close enough to touch the Spellheart in order to channel the spell through it.