Isolate the abstract elements of the works and identify which are ideas and which are expressions.
Filter out the non-protectable elements from the products.
Compare the remaining elements.
In determining substantial similarity, the intended audience must be considered. If something targets an expert group, it should be tested if it is substantially similar to that group, not to lay people.
For profits, you can get actual damages plus all profits attributable to the infringement, and the burden of showing actual damages shifts to the infringer. (You get all profit unless the infringer can show just actual damages.) 17 U.S.C. § 504(b).
For statutory damages, you can get $750–$30,000 per work infringed upon. (not per infringement) If willful, it can go up to $150,000. 17 U.S.C. § 504(c). You can also get attorney's fees and possibly punitive damages.
For statutory damages, you have to have registered an unpublished work or register a published work before or within three months of publishing it. 17 U.S.C. § 412.
Statutory damages are subject to the innocent infringement defense. If the infringer had no reason to believe that his acts constituted copyright infringement, the damages may be reduced, to a minimum of $200. This defense cannot be raised if there was a notice of copyright with the work.
A protected purpose is something such as criticism, comment, news reporting, teaching, scholarship, or research.
Parody is a subset of criticism or commentary, and thus parody works must direct commentary or criticism at the work. Just being a parody or funny is not a protected purpose. The leading case on the matter is Dr. Seuss Enterprises v. Penguin Books.
[O]ne who distributes a device with the object of promoting its use to infringe copyright, as shown by clear expression or other affirmative steps taken to foster infringement, is liable for the resulting acts of infringement by third parties.MGM Studios Inc. v. Grokster, Ltd.