Username registration is live on the network right now, but will be publicly available in the Dash wallet very soon! Download it now and update it when this feature goes live:
https://www.dash.org/downloads/
More advanced users can use the Dash Evo Tool, which lets you do all kinds of advanced functions:
https://github.com/dashpay/dash-evo-tool/releases
Here's a guide for how to use the Evo Tool:
https://dash-docs--401.org.readthedocs.build/en/401/docs/user/network/dash-evo-tool/index.html