Acids-Bases....?

Question

To make a buffer, you have to make it so the pKa of your weak acid is going to equal the pH right? (50 % dissociation)
My questions are:
(1) How do you do this....can you just put a solution of your weak acid into an already existing solution that has a pH close to that of your weak acid's pKa?
(2) If so, how does the mixing work, stoiciometrically? Like let's say, you want to mix 1 liter of 0.1 M of your weak acid into an already existing solution with a pH you want. (I'm thinking about this kind of in terms of biochemistry, like when somethng acidic enters a cell, rather than traditional methods of making buffers, as by titrating). Can you give me an example? using math, numbers, etc?
Because I'm thinking, if realistically, you add 1 L of your weak acid solution, to another liter of another solution, you'll have 2 liters. Wouldn't that change concentrations of either solution, or the buffer...if you made one? So this is where I'm confused, and need example

Answer 1

The amount of water in a buffer solution is irrelevant - what is relevant is the ratio of concentrations of acid and salt.
You can make a buffer either by mixing measured quantities of acid and salt (in any amount of water) or by part-neutralising an acid with a base.