serial dilution

i researched serial dilution, and it basically means diluting a liquid in water or any other solution. That seems to be what i have planned to do for my srp (see previous blog).
I'm still unsure on how to manage the experiment (Ms Zhang- do all the dilutions have to be done at the same time to make it a fair test?) & i still need to work out the variables; so far this is what i have:

• independent variable- the different types of milk
• dependent variable- the amount of different colonies of bacteria
• controlled variable- times that the milk is diluted; no. of days that diluted milk is kept in agar plates.

I also found the 'recipe' or method for making agar plates, it looks really long, though.. i found another site that doesn't involve getting nutrients, but instead using an agar tablet to make it. and i have to make sure i use the EXACT amount of everything when i repeat the experiment. This SRP may take a little longer than i thought to plan.

Ms Zhang- if i make the agar plates myself, would i have to include that in the time of doing my experiment (2wks-6wks)? And the sites, are they the correct methods?