First you need to unlock a "secret exit" by defeating certain blue blob three times.

Then you will find at end of first boss fight a door. To unlock it you need a key.

After that enter a entrance in middle of room

In 2nd chapter you will see white fire and mirror room,enter to white fire you will turn into "lost" Enter mirror and go to treasure room, pick up item

You will see again an entrance, bomb it two times to enter next alt floor

In 2nd chapter you will see yellow buttons Press them. You will find room with minecart and yellow button. Once you Press all of them enter cart and grab 2nd item

You will see yet another door, it will ask for two hearts after you will give them you can enter third alt floor.

In 2nd chapter you won't have to collect item, instead after mom fight you will see another door enter item that you got to door (if you got that item you will figure out how to enter it) , you will fight harder version of moms heart kill it and go to corpse