First you need a transmitter. Something like the following product.
http/www.monoprice.com/product?p_id=10601
There are other choices as well.
Then you will need an RCA extension cable with RCA style connectors (red-white pair) on each end. You may even have a RCA extension cable around somewhere.... If so, then you can sacrifice it accordingly.
Cut the extension cable in half and strip the installation off of the cut ends.
The cut ends will go into the wire points on the back of the hifi unit. The uncut ends (with the RCA connectors) will go to the transmitter.
Something along those lines to match the connection(s) between the hifi and the transmitter.
Just need to get the correct wire gauge and be careful with the cuts and stripping.
Lots of wiring diagrams can be found on line so look around a bit.