Rotoscoping is a visual effects technique that can be performed on photographs in Adobe Photoshop. It involves cutting a subject out of an original photograph, and then replacing that background with a new one. It's a somewhat tedious task to perform, but if it's done right, the results can be powerful. You can use this technique to create a convincing image that never really existed, like a UFO flying over a neighborhood.
So, you've searched far and wide and you found two photographs that you're going to use for the rotoscope effect. Open the photo that has your subject, the UFO, in it. There are two tools that you can use in Photoshop to cut the UFO out. The 'Magic Lasso' tool would be very fast at performing the task, but it isn't always accurate and could cut off some of the subject. It could easily get messy and require that you start the process over again.
A safer bet is to use the 'Pen' tool. Zoom in on your image and take your time with the pen tool to cut the image out, while preserving every detail. When you rush you make mistakes, and mistakes look sloppy. Take your time and produce quality work.
Once you have cut the image out go to 'Paths' tab, then right click (command click) and choose 'Make Selection'. Leave the feathering at one pixel. Next, go back to the 'Layers' tab and hit 'Control' + 'J' to create a new layer with your cut out subject in it.
Before you rotoscope, you want to make sure that the subject is ready for this effect. Click on the new layer to view the subject on a transparent background. If part of the old background is still present, then use the 'Eraser' tool to get rid of it.
Open the file for the background in Photoshop and have the photograph be in it's own separate window. Go back to the photo with your subject, and make sure that the cut out layer is selected. Click on the subject and drag it into the new photo. It will be it's own layer.
The subject probably will not look like it naturally belongs in the photo at this point. Because you dragged it into this new photo, it will be its own layer. There are a variety of effects and filters that can be applied to the subject to make it blend in.