How do I put a manual switch to a cooling fan on a 94 civic

You could run a pair of wires to heat sensor on radiator, To make sure right sensor(s),
With ignition on, then, jumping when cool, or unplugging while hot, should indicate correct sensor(s). The 2 wire keeps simple, and on same fuse, easily restorable to original.
Could use relay at fan, with 1 small wire for control. Are you wanting to override on or off?
Different temp sensor may help. Timed off, may keep from having a dead battery, unless it was ignition switched. Any more info?