The first piece of bondage furniture most people think of is probably the St. Andrew's Cross. The ubiquitous "X" cross is rather simple to build, and makes a sturdy and useful place to restrain someone. As simple as the project may be, there are a number of design considerations and optional things that can be done with a cross. As I started outlining them for the page, the number of graphics and amount of text grew and grew! The cross project is now broken across this and two other pages.
Accurate protractor or good guesswork; router, table saw, or circluar saw if you're brave. Note that you could eliminate the lap joint to get around the latter requirement, but the result is quite inferior to use.
The basic cross is depicted in the overview diagram below. You have choices to make about desired height/width of the main body (the X), which determine both the angle of opening (between the two pieces) and length of lumber required. If you're making a generic piece or play with people of typical size, I suggest the dimensions in the "common cross" diagram. They assume 2x6 lumber.
You also need to pick a type of rear support, and design it to specifics. See the Rear Leg(s) section of the Basic Cross page for some ideas. Finally, look at the Accessories page and see if you want to enhance the basic design. Plan everything before you start making sawdust.