Take the manifold apart and turn the head upside down. You'll see the four green elbow connectors. They remove easily if you squeeze the ends together with needle nose pliers or tweezers. Remove as many as you'll need for supply lines. For example, if you only need three lines, remove three elbows and leave the fourth one in as a blocker. Once the elbows are removed, take your lengths of 1/4" tubing and cut a sharp angle at the end of the tubing as shown. This helps greatly when inserting the tubing into the manifold holes as it fits very tightly which is what you want it to do.
 
Aeroponic Manifolds
Two simple aeroponic manifolds that work well - 11/8/2008

We got talking about manifolds in a forum thread, so I decided I would show two different kinds of manifolds I use on my Aeroponic machines. The first one is exactly what I'm using now on the TI Smartbar and Smartlamp tests. These work well and produce good pressure (with a slight modification of the 4-way manifold). I've been using these for a long time with no problems. I buy these at Walmart for about $4 in the Garden Center, but can't seem to find it on Walmart online, but here is a link to one  Orbit Manifold The manifolds are still available in Walmart Garden sections as I saw them a couple of weeks ago.  

The only trick is removing the green elbows and inserting the 1/4 black tubing (found at any Hydroponic shop). For aeroponics, it's necessary to remove the elbows as they cause too much restriction and work much better with the modification.

Push the 1/4" line into the manifold hole. Turn it over, grab the tubing with pliers and pull the tubing through until the entire angled cut is through the hole. As you'll see, the tubing is very snug and will not pull out very easily, nor does it leak (it's inside my tank attached to the pump anyway if it does). Cut the end of the tubing square removing the angled cut.

The other type of manifold I use with success is made up of various PVC parts. I use these in my one piece grow boxes. All parts are 1/2" found at any hardware store. The elbows and tee are the types with one screw-threaded end. Of course there are many other shapes you can use and larger/smaller designs, but the below photos will give you the basic idea. I don't glue the parts together as just pressing them togther seems to be fine and it's easier to clean if they come apart. One thing to be careful with is the drill you use for the sprayer holes. It needs to be slightly smaller than the sprayer threads so the sprayer screws in snugly. I use a thread tap, but if you find the exact size drill, tapping isn't necessary. Unfortunately, I don't know the exact sizes of the drill and tap I use as it's not stamped on them, but they are standard sizes I found by trying them on scrap PVC until I found the right ones. I find on the bottom manifold, a 185 GPH pump works fine
for four sprayers.

Pull the tubing back until it's flush as shown. Repeat for the other elbows. Reassemble the manifold omitting the small strainer as it just blocks pressure and clogs up anyway (I use nylon stockings over my pumps for filters). Screw a plastic 1/2" pipe nipple into the bottom of the manifold, attach the other end to your pump. You now have a four-way spray manifold. I use the small green 360 degree sprayers sold at most hydroponic shops like these: 360 Degree Sprayers

 I usually make a bracket out of plastic inside the chamber by drilling a 1/4" hole, sliding the tubing in, and then screw the sprayer right into the tubing end. Because of the length of tubing I run, I find a 396 GPH pump works great for up to four sprayers. I use one sprayer per unit on the aeroponic boxes you see in my grow test pages.