Helen, I’ve worked with enamels a little. Here are my answers to
your questions. I’m sure the more experienced enamelists, especially
those with lots of torch-fire under their belts, will correct any
errors in my response 
The enamels come ready ground and I'm assuming that the colours can
be mixed. Am I right in thinking this?
No, enamels cannot be mixed like paint. If you mix, say, blue enamel
and yellow enamel, you get a very pointillist version of green, if
you get green at all. Mostly you’ll get a disappointing glop color.
That said, there are enamels ground finely enough to be used as
paint, and those can be mixed to create hues and shades. I’ve only
seen one variety of these, a set of Japanese leaded enamels I bought
from Enamelworks here in Seattle.
Using the torch, I'm presuming that I'll need to flux the piece. Do
I use flux where the enamel is to go or would I just paint it on
the bare surfaces of the metal?
Enamel actually melds with the metal at the very topmost molecular
level. Anything that gets between the metal and the enamel–dirt,
flux, paint–will interfere with required adhesion, and your enamel
will fall off. The surface of the metal must be absolutely clean and
grease-free before you apply enamel to it.
As for soldering, when constructing a cufflink which is to be
enameled, would I solder the parts together before or after doing
the enameling?
Any and all processes that require heat must be done before
enameling. Also, you must use a special solder called IT solder which
is formulated to withstand the higher temperatures the piece will
undergo during enameling. IT solder has a higher melt point than hard
solder. I’ve never tried enameling on a soldered piece before, but
I’ve heard tales that even IT solder sometimes fails.
The solution here is to enamel your logo piece and then bezel set it
in the constructed cufflink.
What do people use to finish the enameled surface?
There are special stones called alundum stones that come in
different grits and are used to grind down the enamel surface until
it is flat. Flat diamond files can also be used. These also come in
varying grits. A couple pieces of critical 1) Do all
your grinding under water; you are grinding glass, and you want all
those tiny glass particles kept out of the air you’re breathing. 2)
Grinding the surface of the enamel dulls it; once you’ve ground the
surface flat, you have to refire to re-establish the gloss.
Make sure you test each of your enamel colors. Torch firing often
produces a different color than expected. I remember a lead-free
“ivory” that torch-fired to a metallic yellow-green. It was a
wonderful color, as long as you didn’t need it to be ivory.
You cannot pickle enamel for very long if at all. Pickle etches into
the surface of the enamel, and then you have to fire it again.
Depending on the amount of enamel you need on the top surface of the
metal, you may need to “counterenamel” the back of the piece. I know
next to nothing about the science involved, but enameling changes the
ability of the metal to flex and expand, reducing the flexibility on
the enameled side. If there is too much enamel on that one side, the
metal will bow and pop! the enamel comes off. Counterenamel is
putting enamel on the back of the piece to balance out the expansion.
Another very good reason for bezel setting the finished enameled
piece.
You don’t say how you are getting the logo created in enamel. If the
logo is anything other than a plain shape with just one color on it,
you’ll have to do either champleve or cloisonne technique, and
neither is what I’d call beginner work.
Before you accept this job, read through a beginner’s book on
enameling. There are several, including one by Dorothy Cockrell in
the UK. There is much more to enameling than meets the eye.