Push the card into the slot. It'll make a click noise and the card will "jump" out.

Congrats! You can now put your new card in and your phone back together. Your card isn't usable yet - in order to use it with Windows Phone 7, you have to make a factory reset.

Factory reset: Once you've put your phone back together, hold down the vol down and the vol up button and turn on the phone. You'll get into a grey screen. Read the text and then perform a factory reset by pushing the vol down button.

Your phone should now restart and bring up the set-up screen (like on the first start of the phone).

If the factory reset doesn't work for you (that means that the phone restarts all the time or that it doesn't start at all) try it a few more times or flash an original rom using a RUU.

The end of the story should be, that your phone's running - with the new microSD in it.