I've installed tail wheels on several already built models. Some were originally nose draggers and some were a replacement of the original tail wheel. One was a conversion from an impossible castering wheel to one that steers

If the design allows, I use the Sullivan as Ken suggested

On the others with a Dubro bracket, I ran a new tailwheel wire up into the hinge line of the rudder. Turn the rudder all the way and with a 90 degree bend in the tail wheel wire, stick it into the rudder with epoxy.