Converting 2 lamp ballast to a one lamp
You don't, unless the ballast label or data sheet  says you can.  

In your case the ballast label says "For 1-lamp operation, individually insulate the yellow leads for 600V" - that is canonical.  Further, it makes sense.  They need to be capped off from each other because there is preheat voltage on them.  

Since you need 4 wires and there are 4 wires left, do the obvious.  The red wires go to the same end, because they provide lamp preheat for that end.   That's what makes it a rapid-start ballast and not instant-start.  Despite the names, rapid-start is better - it starts more reliably in the cold, and lamp life is much, much longer - because it starts the tube using its preheat feature as intended. 

This does not mean you can do this 1-lamp trick for any rapid-start ballast.  Many do not support it.  You got lucky, don't chance it again - check the label or data sheet. Labels typically list some of the tubes supported; data sheets list every one.