Quick setup
Already played? Answer two quick questions and the guide fills in everything they imply. For example, beating Mother means you also beat Mom, so you won't be told to do that again. Your answers save as you tick, in this browser only.
1. Which bosses have you beaten?
At least once, unless it says otherwise. Any character and any mode counts.
2. Which characters do you have?
Isaac is there from the start, so he isn't listed.
Tainted characters
3. Completion marks (optional)
Most achievements are rewards for beating bosses with a particular character. Open a character page and click the marks you have on their in-game note. Everything else can be ticked on My progress or on any guide.