Hi
I will give you a run down on how i achieved this using Yonomi. Download the app for Yonomi and add the skill to Alexa before starting.
First of all i am running the whole thing to start with on my iPhone and have loaded the Alexa App. the Yonomi App. and the Harmony Hub App.
First of all go into the Harmony app. and add all your devices you are wishing to control.
Then create a new 'activity' for what you wish to do i.e. Watch TV or Sonos Music or similar type names and then add into this activity your devices and the actions you can take i.e. Samsung TV turn on, Sky Box turn on, Set TV to HDM1 (or whatever source you have connected), Sonos Player turn on, Sky Box change to channel 115. I tend to do this on my computer rather than the iPhone as it is easier to do. Some the items may be easier to add in after you have created the initial activity switch on routine (i.e. channel number) by going into customise routine.
Once this is complete then you need to go to Yonomi and add Harmony Hub to the 'Things' page then in Routines add a Watch TV and a Watch TV off routine (you need both for it to work) it will prompt you to add the Harmony Hub Start Watch TV and End Watch TV activities.
Once you have done this then go to the Alexa App and Smart Home and discover new devices (leave all apps running on your iPhone or iPad or whatever device your are using) and you should get after a few seconds the Watch TV Routine and Watch TV off Routine (Yonomi) added as a device.
You should now be able to say 'Echo turn on Watch TV' and the Harmony Hub will do the business.
If it is a Music device (you mention hifi) you are wanting to set up then his suggest you don't use 'music' as the key word but use a device name i.e. Sonos, Stereo, Living Room, Technics, etc. as Echo gets confused with what music you want hence she played Mumford & Sons. Much Better if you can say "Turn on my Stereo" or something similar she can't confuse with Amazon Music or other music services.