While HHO generators (hydrogen-hydrogen-oxygen generators, aka "hydrogen boosters") are fairly easy to build and are undoubtedly good for fuel mileage, the fact is that they produce a dangerous mixture of gases. Beyond the fact that hydrogen itself is explosive in an open atmosphere, when it is combined with pure oxygen it becomes the main ingredient in a fuel/air bomb. Flashback arresters are used to keep engine backfires from igniting whatever oxyhydrogen gas is in the lines and the booster.
Flashback arresters come in several different varieties and they all do essentially the same thing: Arrestors keep burning gases from blowing through the tube. The three basic types are the valve-type (which uses suddenly increased air pressure in the tube to close a one-way valve); filter-types (some use bronze wool and others use permeable stone); and water bubblers. Home-made valve-type arresters can be a bit difficult to calibrate, so if you're going with a valve, you'd be safer just buying one.
Filter-Type Flashback Arrestor
You'll need a 2-inch-by-1/2-inch brass couple (with threads on both ends), two 1/2-inch brass ends with barbed fittings and two 1/2-inch outside-diameter, cylindrical aquarium stone bubblers (all of which you can find at Lowe's or Home Depot). Drill 3/4 of an inch into one end of the centre of the bubblers' plastic tips (the goal is to expose the bubblers' inner channel), and coat the outside part of the bubbler on that end with silicone. Insert the siliconed end of each bubbler into the brass end pieces, and coat the coupler threads with Teflon tape. Screw the assembly together, and you've got yourself a DIY filter-type arrester.
There's an old joke going around in the HHO community that whoever came up with the bubbler arrester was probably using it for something else beforehand. The fact is that bubbler arresters do resemble (in both form and function) those water-filtration pipes you might have seen in the local "tobacco accessories" shop.
To make a bubbler, you'll need a clear plastic cylinder (approximately 3 inches in diameter and 8 inches tall), a 1/2-inch outside-diameter, 12-inch long plastic tube, two 3-inch inside-diameter PVC pipe caps and a few fittings. Cement the upper an lower radiator caps to the tube, and drill two 1/2-inch holes in the top. Run the plastic tube (connected to the HHO cell output) through the cylinder's cap to the bottom of the cylinder (like a drinking straw), and connect another tube (the HHO output tube) to the cap's second hole. You might want to consider using brass fittings for all your connections; they'll seal well and make maintenance easier. Fill the cylinder with water (about 8/10 of the way full), and connect the output tube to your engine intake. The water will stop the flame front before it reaches the HHO output tube. Check the Resources section and you'll have an idea of what you're building.
Mount the bubbler lower than your HHO cell to prevent any water backflow into the cell. Everything you'll need to build the bubbler you can find at your local hardware store.
Do not under any circumstances do what you've seen online and use a glass jar or cylinder; the point here is to contain a potential explosion, not to build an engine-mounted glass grenade.