Here are some detailed instructions from Amazon Customer Service:
Before you begin:
Set your Echo device and Bluetooth speaker at least three feet (one meter) apart.
For best results, use a Bluetooth speaker certified for use with Echo devices. For more information, go to Certified Speakers and Supported Bluetooth Profiles for Echo Devices.
Make sure you can connect your speaker to other Bluetooth devices, like a mobile phone.
Turn on your Bluetooth speaker and turn up the volume.
Disconnect other Bluetooth devices from your Echo device. Echo Device can only connect to one Bluetooth device at a time.
To connect a Bluetooth speaker to your Echo device with a screen:
Turn on pairing mode on your Bluetooth speaker. Refer to your Bluetooth speaker user guide for more information on this.
Say “Go to Settings,” or swipe down from the top of the screen and select Settings.
Your device enters pairing mode. When your device discovers your Bluetooth speaker, the speaker appears in the list of available devices. Select your Bluetooth speaker and follow the instructions on the screen. Your devices connects to the speaker and Alexa then tells you if the connection is successful.
To connect a Bluetooth speaker to your other Echo device:
Turn on pairing mode on your Bluetooth speaker. If you don't know how to do this, refer to the user guide for your Bluetooth speaker.
Open the Alexa app and select Settings.
Select your device, and then select Bluetooth > Pair a New Device. Your Echo device enters pairing mode. When your Echo device discovers your Bluetooth speaker, the speaker appears in the list of available of devices in the Alexa app.
Select your Bluetooth speaker. Your Echo device connects to the speaker and Alexa tells you if the connection is successful.
In the Alexa app, select Continue. Your Bluetooth speaker is now paired with your Echo device.
Tip: To connect to a previously paired device, say, “Connect.” Your Echo device connects to the device that was last connected.