Ummm, what do the directions say?
You read the directions didn't you?
And you found out what all the parts are for?
Including those plastic thingies (shields or anchors) that are usually packed with the screws provided?
Just take an awl or small phillips screwdriver, and using the baseplate, or the layout diagram included with the instructions, and place it where the instructions tell you to put the smoke. Then, using the awl or screwdriver, push in to create a hole that is smaller then the anchor and tap in the anchor.
Screw in the baseplate using the screws provided and put in the battery, if it is not already in the smokie and then put the smoke detector in place.
Didn't mean to be quite so snarky, but, it is in the instructions.