The paper makes the Diffie-Hellman tree they use pretty easy to grok: https://eprint.iacr.org/2017/666.pdf

For those interested in ART, there is an IETF working group attempting to standardise it as the "Messaging Layer Security" protocol. Their website [1] has a draft protocol document [2] which explains the ART scheme from a concrete engineering perspective, which I find easier to follow than the original ART paper.

[1]: https://mlswg.github.io/

[2]: https://github.com/mlswg/mls-protocol/blob/master/draft-ietf...