Cover the floor and furniture with plastic or painter's canvas so that paint doesn't damage your furnishings.
Line the stencil up where you want it to go on the ceiling. Start in one corner if you will be stenciling the entire ceiling.
Lightly spray the back of the stencil with spray adhesive to help hold it in place. Remember that you will be working against gravity so the stencil won't want to hold in place.
Stick the stencil to the ceiling and then tape with painter's tape for extra support.
Roll the paint roller over the stencil, being careful not to press too hard. You also don't want to use too much paint on your roller at one time because this presents the potential for paint drips.
Wait for the paint to dry and then add a second coat of paint. Wait for the second coat to dry and then remove the stencil.
Line the stencil up next to the first painted design and repeat the process until you cover the entire area. If you wish to paint the whole ceiling with the design, lining the stencil up correctly is important, but most stencils should have some kind of mark to indicate where to place the next stencil.