Remember there are 2 write-in campaigns going on: One for Norman Seigel for Public Advocate and Paul Wooten for Brooklyn DA. DFNYC has a great intro at How to Cast a Write In Vote | Democracy For NYC.

The explanation though works with one set of voting machines. This is what Mole333 had to say on a recent email :

Begin voting as usual (lever to the right, etc). For all positions you want to vote for normally, just flip levers as usual. To write in, look at the LOWER RIGHT HAND SIDE of the machine for a silver lever marked "Release lever." Flip that and leave it flipped. This releases the little numbered sliding doors along the LEFT HAND SIDE of the machine.

Find the numbered sliding door that corresponds to the number of the office you want to vote for and open the door. There will be a slip of paper behind the door. Do not remove the paper, simply take a pencil (there should be one to your left in a little cup on the machine) and write the name of who you want to vote for. When you are done writing, simply close the little door.

That's it.

When you have flipped all levers and written in all write-in names, leave the silver release lever filpped (do not flip it back) and pull the big lever back to the left as usual.

Just because you are unhappy with the choices it does not mean you have to stick to the choices offered on the ballot.

Use your vote to write-in your dissent. It's the democratic thing to do.