well its best to bolt down the switch and clear all surrounding wires etc as best you can. Presolder the diode up on the bench .. Set up the wires after pretinning them so they rest where you need them heat and hold...should be that easy. Might have to add solder if a little light ..but thats it.. no easier way my friend.
Good luck

For wires that don't want to stay in place I keep one of these in my toolbox to hold the wire... Clamp one end to some bracket, the other end to hold the wire in place. Shrink wrap tubing on the alligator clip so you don't damage the wire insulation. https://www.etsy.com/transaction/1036609892

Sometimes I use my hemostats to hold the wires and lodge them into a wire loom/harness to hold them fast while I do the soldering and holding.

That thing is cool beans...

yes exactly. You should have already pretinned the terminals where the wires go when you soldered the diode on as well. Really makes things easy ...just pretin, get everything ready....heat and hold.

Just don't solder the switch 'upside down' to prevent solder or rosin from getting inside the switch body and mucking things up.