Since 1988, Canon's EOS series of SLR cameras have offered photographers high-quality photography, first in film and later in digital formats. EOS cameras have a common lens mount for Canon lenses made after 1988, but aren't outright compatible with FD lenses made prior to 1988. The FD lenses had a smaller lens mount and need an adapter to be attached to an EOS camera. Canon produced two different adapters that can be used to bridge the gap between the FD mount and the EOS body.
Locate and purchase an FD lens adapter for your EOS camera. There's no way around this, if you want to use an FD lens, macro or otherwise. Canon made an adapter in the late 80s that was expensive then --- $250 --- and is more expensive now --- $1000 --- according to Bob Atkins's photography blog, as of January 2011. Atkins describes a few generic options, but ultimately recommends the ELEFOTO adapter, which has optical elements that allow the EOS camera body to focus manually through the FD lens.
Attach the lens adapter to your FD lens by threading the FD lens into the wider end of the adapter. Hold the lens release button and turn the lens counter-clockwise to remove the current lens from your EOS camera body. Then attach the lens and adapter assembly into the camera body. Macro FD lenses that work with the lens adapter include the 200mm 1.8 L, 200mm 2.8, 300mm 2.8 L, 300mm 4.0, 300mm 4 L, 400mm 2.8 L, 400mm 4.5, 500mm 4.5 L, 600mm 4.5, 800mm 5.6 L, 50-300mm 4.5 L, 85-300mm 4.5 and 150-600mm 5.6 L.
Turn on your EOS camera. Depending on the model and medium, the process of using the FD lenses will vary a little. For digital SLR cameras, rotate the selection dial to M for full manual shooting or Av for aperture priority mode. Note that the FD lenses were manual focus lenses and that the autofocus drive of your camera will have no effect on them. Switch the focus dial beside your lens to manual. Take some test shots to see if the exposure is correct without any specific adjustments. Compensate the exposure and white balance as necessary.
Spin the focus dial on the lens to bring your shot into focus. Some EOS film cameras had a focus dot that would turn green when your shot was in focus on manually mode, but this feature generally doesn't work with FD lenses. According to Bob Atkins, another FD adapter was released in 2006 that has a microchip to pass focus information to your EOS camera body. Practice will make perfect with manual lenses, so go out and experiment.