You must type a lot. I wish I could tell you how often and when exactly to type, but I can't. On Day 3 when you speak to Bailey, always pick the second conversation option. Also when Michael attempts to type, his attitude towards it gets more negative. He will eventually say that he doesn't want to touch or look at the typewriter. If you get an entry in Michael's journal, I am not Michael Arthate you will know that you have triggered the alternate ending. You must still make the amulet, etc. But when you go down the fireplace and open the door, if you have the alternate ending, you will know right away.

a number of times each night near 6 to 7 pm. And when you call Bailey --- use the 2nd questions (the alternate questions --- get it???} in each dialogue tree.
Then just befire you go down into the grating --- go back up
and type until you see a strange entry into the Journal....
{one will be about throwing the typewriter out of the windoww nd the other will be about Michael stating that he is not Michael)
then go downstairs --- keep in mind that he Easyer Egg for the Bank telephone conversation will not work with this ending.

more on the alternate ending. I don't know if this is really a spoiler or not, but I typed a great deal at an earlier series of times and have, in the journal, all the comments Inferno mentions. Be aware that I just left the chapel and have lots to do before I open the grate..