Making MES is the same as making most any buffer. Essentially you titrate a fixed amount of MES in either its acid or basic form. For MES the starting point is usually the free acid which you titrate with NaOH to the correct pH. You need to decide what concentration you want to have for the buffer (25 mM or 500 mM—whatever you need so long as you’re not above the solubility level for MES; caveat: the fee acid of MES isn’t all that soluble. It is not uncommon to start with a slurry of MES which dissolves as you add more base to the solution). Once you’ve decided what concentration of MES you need, you then decide how much of it you want to make (say 100 ml or 1000 ml—whatever you need). You need to know the MW of the free acid of MES and then you’re all set.

Say you want to make 1 L of 100 mM MES at pH 6.0. The MW of MES (free acid) is 195.2. 1 L at 0.1 mole/L requires 0.1 mole of MES or 19.5 g of MES. Put 19.5 g of MES in something like 500 ml of water, put in a pH electrode (the starting pH will be something below 4.0 probably) and titrate to pH 6.0 with either 1 N or 10 N NaOH. Once you’ve got the solution to the correct pH, transfer it to a graduated cylinder or volumetric flask and dilute it to a final volume of 1 L and you’re done. Different volumes or different desired concentrations of MES will require the use of different amounts os solid MES, but the process will be the same.