I’ve used that switch a lot too. Depending on what speaker loads you are using, connect the correct color secondary leads from the OT to each side of the switch and then connect the remaining switch lug to the speaker. You can check it with your meter. I think the kit suggested wiring when using a 4 & 8 ohm speaker load and the Hammond 125ESE OT is to use the Yellow and White secondary leads.

To learn about how the OT works with the primary input impedance (from the power tube) and how it relates to the correct secondary impedance. DH’s book covers it as does Merlin Blencowe’s great website: http://www.valvewizard.co.uk/se.html

Using the correct speaker load is important but not exact. Fender has gotten away with 100% mismatched impedance (when using the ext speaker out) from the beginning. Some designs even use impedance mismatch intentionally. Using the correct load is easier on the OT and give the most output.