Depends on the gender for the specifics but you need a Y-splitter with the correct ends for the two audio sources which are merged into the end to go to your amp, receiver, powered speakers or whatever you are using.
For example this one was right for me:
https://amzn.com/B000067RC4 Because the connected on my amplifier is female 3.5mm stereo or RCA stereo, the stereo 3.5mm plug is simpler. Then you have to connect one of the ends, on this one it's female, between your Echo and the Y-connector. In my case it's something like this:
https://amzn.com/B000SE6IV8Incidentally, I use this amplifier:
https://amzn.com/B0049OI08WIt's nice because it has a white range of connectors and best of all you just leave it on, it goes into low-power mode when not receiving a signal.
And in case you're interested, it feeds into these speakers:
https://amzn.com/B00009P2YQ I've got about 6 of these with speakers mounted in wall or ceiling. Now one of them has Alexa so you can say "Alexa, stream WLGE", adjust volume with voice, no quickness that plagues Airplay sometimes when moving around the house, and no draining your iPhone or iPad battery streaming audio.
Mitch