No problem... good question.
SPDT stands for Single Pole, Double Throw. That means there is one set of contacts (single pole) and two possible contact positions (double throw). An SPDT switch can be on-on, on-off-on, or off-on-on depending on how it's built. In your case, I would recommend an on-on switch, although an on-off-on switch will work just fine (you'll just never have a need for the off position).
The picture above was of a DPDT switch, meaning there are TWO sets of contacts (Dual Pole) and two positions for them to make contact.
Ordinarily, a DPDT switch would be used to use a single power source connected to the center terminal(s) to power item A or item B depending on which way the switch is thrown. For our purposes, we want the switch to select power source A or power source B and send that power out to the transfer case. Essentially we're wiring the switch up 'backwards', but it works just fine for what we're asking it to do. Make sense?
In the sketch above, if we call A the top terminals and B the bottom terminals, when we throw the switch to the A side, we connect the existing brown wire to the transfer case clutch coil. When we switch it to the B side, we connect the fused 12V power source to the TCCC, locking the transfer case in at 100% duty cycle.
(On a side note, even though the case is locked, on a 95 and 96, the dash selector switch still needs to be set to AUTO for the front axle to lock in)
Make sense?
Hope that helps!