Check your class or spellcasting feature to see which ability score it uses
Use the listed spellcasting ability for that class or feature
Calculate the ability modifier from that ability score
Add proficiency bonus if the feature says to add it to spell attack rolls or spell save DC
For multiclass characters, use the spellcasting ability tied to each class’s spells
For spells from items, feats, or special features, use the ability specified by that source
If no ability is specified, follow the rule set or feature description that grants the spell
Spell attack modifier = spellcasting ability modifier + proficiency bonus
Spell save DC = 8 + spellcasting ability modifier + proficiency bonus