Help in making molar solutions

[Corona90]

Help!

I am a microbiologist, and we recently started making our own reagents for
a molecular biology project (pulsed-field gel electrophoresis used for
epidemiological purposes).  I am having problems figuring out how to make
certain buffers and solutions with some of the published protocols.  For
example, one of the solutions we use is called TEN solution and has the
following formula:

40 mM Tris-Cl, pH 7.5
1 mM EDTA, pH 8.0
0.15 M NaCl

I remember enough of chemistry to make up separate molar solutions of
each, but what's the secret to combining them?  If I mixed them in equal
amounts, wouldn't their overall molarity change?  I am missing something
fundamental here, and could really use the help.

Thanks,

-gary cage
 AZ State Laboratory
 Dept of Microbiology