Depending on your objective, there are various ways to run Nimbus in Ethereum 2.0 mode.
The reference documentation is the Nimbus Book: it explains all the ways in which you can play with Nimbus, and use Nimbus to either monitor the eth2 chain or become a fully-fledged validator.
We encourage you to check it out here: https://nimbus.guide
The latest Eth2 testnet, Prater, is now open to the public.
Prater’s objective is to ensure that the network remains stable under a higher load than we’ve seen so far on mainnet – the genesis count for Prater was 210k (almost double the size of the Beacon Chain Mainnet).
For more, see the Prater page of the Nimbus guide.