Hello! I bought a doukake/tanjin cap set online, but it arrived without any strings to attach the doukake. Is there a certain kind of string I should use? Should it be stretchy or not?
I use a thin green leather cord that matches my awesome green lacquered bachido doukake. 
You can find it in any color online for bracelet making.
Anything non-abrasive will be fine.
I’ve used real leather, fake leather, string from the dollar store, shoe laces, paracord. Anything works. I know folks that use a bit of sticky tack (or the like)
For the tenjin cap you may want to add a small cushion, similar to the rubber bit on the bottom of the dou to help it stick.
thank yall, i picked up some leather cord yesterday. good tip about the tenjin cap!