TONS of ways to diffuse C02... Some are MUCH better than others... I will list a few of them That I tried and seem to work out pretty well...
1) Set the tube so your HOB filter will suck the C02 bubbles into the intake... Impeller chops up the C02 and disperses it into the water fairly well.
2) Put the tube into a power head... Power head blows tiny bubbles into tank which dissolve pretty well.
3) Build a diffuser out of a power head and other things to 100% diffuse the C02 into the water better.
I will list some links that I have used that were very helpful...
Good info on C02 --->
www.rexgrigg.com
DIY C02 contruction --->
http://www.plantedtank.net/articles/DIY-Yeast-CO2/7/
More DIY C02 stuff --->
http://fish.cecolts.com/pics/co2.html
Measuring C02 in tank --->
http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm
Even more good info ---> http://www.qsl.net/w2wdx/aquaria/diyco2.html <----- This link is THE BEST!!!
That should get you started... There is a TON of info out there about DIY C02..

Just google it...