Why?

The manifold is used in the wort boiler after boiling (and perhaps cooling) to rack the wort off the hops and trub. It's less of a hassle than the typical racking cane with scrub pad on it's end and is much more effective. Since it's located at the very bottom of the boiler (a 8 gallon enameled pot), it gets all but a cup or so of the wort.

Caveats

• I always use whole hops. I've no idea how the thing will work with hop pellets but suspect it wouldn't work nearly as well as it does with whole hops.
• I usually use an immersion chiller and whirlpool the cooled wort. The few times I've used a counterflow chiller, the whirlpooling didn't work nearly as well due to the convection currents. The run-off was a bit cloudier, slower and more prone to clogging than when an immersion chiller was used.

• Get 3/8" OD copper tubing. I like refrigeration tubing. I bend free-hand. If you want tight bends, use a spring type tube bender that fits over the tubing or pack the tubing tightly with fine dry sand, crimp the ends and bend.
• Bend the loop and gooseneck to fit your boiler. The loop should be just slightly smaller than the ID of your boiler and the gooseneck should just clear the top of the boiler. Note than the free end of the loop overlaps the part of the loop above it. This will later from a solder joint to make the loop stronger.
• Crimp the free lower end of the loop shut. I used a hammer and anvil but pliers would also work. Form the crimped end into a concave cross-section so that it mates well with the part of the loop above it. I used a piece of 3/8" steel rod on and anvil and a hammer.
• Clean the mating surfaces (a green scotch-brite pad is great for this), apply flux, clamp or temporarily wire the joint together and solder with lead-free solder. Be frugal with the solder and wipe the joint of any excess. After it cools, clean flux and residual solder from the joint.
• Saw the slots. A jig for holding the loop is highly recommeneded! Mine was made from a scraps of 3/8" thick wood nailed to a piece of plywood. I used a Dremel MotoTool equipped with an abrasive cut-off disk to cut the slots. A hack saw can also be used but one must use a very fine blade. The slots are a bit over 1/3 the way through the tubing and are spaced about 3/8" on center.
• Remove exterior burrs from the saw kerfs with sandpaper or a fine and follow with a scrubbing with a scotch-brite pad.

• Fasten a 4-5' long piece of 5/16" ID x 7/16" OD vinyl tubing on the end of the gooseneck- this joint needs to be air-tight. I use a couple of wraps of wire to secure the joint. Put a valve in this tubing close to the discharge end. The valve should have the ability to shut tightly and also throttle flow. I like the external nylon clamp type valves. I usually pre-sanitize the tubing in Iodophor but it probably isn't necessary.
• About 5 minutes before the boil is terminated, put the manifold in the boiler. Protect the vinyl tubing from the burner's heat. I use a 6"x16" piece of aluminum flashing that's bend into a long tube and wired to the gooseneck. Allow the tubing to droop in a U shape with the free end above the level of wort in the boiler and siphon it almost full of boiling wort. For starting the siphon, I recommend a rubber bulb type hand pump that's intended for priming outboard boat motors. Put the free end into a big Pryex container and lower it until it fills with wort then raise the container to let the wort siphon back into the boiler. Repeat several times then shut the valve to trap the siphon. Instead of this method of sanitizing, I suspose one could just use an iodophor bath. With my large manifold, finding a big enough container to do this in is problematical. Cover the free tubing end with aluminum foil.
• I cool the wort with an immersion chiller and then whirlpool with a sanitized spoon. To whirlpool, remove both the chiller and the manifold. If the manifold is left in the boiler, the whirlpool is greatly impeded by the manifold's riser and the process is just about worthless. I used to put the manifold in a trash bag that had been sanitized in Iodophor- this was a hassle. Now, I just hang the manifold up from a string fastened to the ceiling. Wait until the whirlpool subsides then a couple of more minutes and gently put the manifold back into the boiler without disturbing the hops and trub which should be piled nicely in the center of the boiler's bottom.
• Put the free end of the racking line into your fermenter or whatever and tease the valve open. I usually rack at about a qt. per min- 20 minutes to get 5 gallons.
• Racking hot wort is more prone to clogging since one is racking through a hop filter bed. A slow flow is the key to avoiding a stuck filter bed. Start the flow very slowly to set the hop filter bed. The first quart or so will be cloudly. Recycle this to the boiler if you can do so in a manner that doesn't aerate the wort. With hot wort, I've found the flow has to be reduced to avoid compaction of the hop bed. Also, with very hot wort, you'll probably note some bubbles in the tubing downstream of the gooseneck. If the connection is tight, this is water vapor- the wort is boiling due to the lower pressure in this part of the system. This boiling also reduces the flow. If perchance the manifold starts to clog, try back-flushing it by putting the end of the racking hose into a container and collecting a bit of wort then raise the container/tubing end in it so that wort flows back into the boiler through the manifold.

A tee could be used at the goosneck-to-riser joint instead of the way Mike's or my manifold is constructed. Finding a 3/8" tee might be a little difficult tho'. Also, if the joints on the tee aren't soldered (particuliarly the "top" outlet of the tee), you'll loose a bit of wort since the siphon will be broken when the wort level in the boiler drops below any non-air type joint. The up-side is that you'll get a more uniform flow. It's flow capability will be increased, but, I don't consider this needed- too much flow is not a good thing if you want clear wort.

Enjoy!