1. Install Avro from OmicronLab.
2. Right click on Assamese Phonetic Keyboard for Avro and select Save Target As or Save Link As to download the keyboard layout.
3. Double click on the downloaded XOBDO keyboard.avrolaout file.
4. Now switch Avro to বাংলা to type Assamese using the keyboard.
The layout of the XOBDO keyboard is as following: