"Wrap" the room in crime scene or police tape by stretching tape from one side of a doorway to another. (Make it look like the room is being blocked off but still allow access.)
Have a "partner in crime" lay on the floor and use the white duct tape or chalk to draw a chalk "outline" around their body. You can either have this person stay there for the party (remember to decorate the person and have them lie completely still) or just leave the chalk outline for effect.
Place drops of fake blood around the body (unless you have carpeting--then skip this step). You could even put some fake blood on the bottom of your shoes and step around the chalk outline to create bloody footsteps. If you have carpet, put the fake blood leading up the steps to the house, and make some bloody footprints also.
Place the fake gun or knife somewhere near the body as the "evidence" of how the crime was committed. If you are the host of the party, you might want to wear a policeman's uniform or dress as an FBI agent to add to the drama.