Mix the suspension to minimize any concentration gradient then use a volumetric dilution. This is often done with pipettes. Sterile pipettes should be used to avoid contaminating the bacterial suspension with other species.

A formula that I find handy when adjusting concentrations is V(1)xC(1)=V(2)xC(2) [V is "volume" and C is "concentration"]. If you have 1 ml of 2.7 * 10^8/ml.and want to dilute that to 1 * 10^8/ml, then 1ml x 2.7 * 10^8=unknown volume x 1 * 10^8. Solve the the unknown volume, which is 2.7 ml. This means that you will have to add 1.7 ml of diluent to 1 ml of your bacterial culture to obtain 2.7 ml at 1 * 10^8. (Note that I have assumed that when you say the "population" is 2.7 * 10^8, that what you mean is that the concentration is 2.7 * 10^8/ml.) On the other hand, if you need 20 ml of bacteria at 1* 10^8/ml for the tests that you have planned, then unknown volume x 2.7 * 10^8= 20 ml x 1 * 10^8. In this case the unknown volume is 20/2.7=7.41 ml. So you would take 7.41 ml of your culture and add 12.59 ml of diluent to it to obtain your 20 ml at 1 * 10^8/ml. As noted, always mix well and pipette these volumes with sterile pipettes.