The steps are very similar for Acala, except that $ACA (Karura equivalent is $KAR) is directly withdrawable from centralized exchanges on the Acala network.
Once you withdraw $ACA onto your Acala wallet (using the same extension above, Polkadot.js), you have the same two options to obtain $aUSD:
$ACA is used for gas fees on the Acala network in the same way $KAR is on Karura.
Note that fees are denominated in $ACA but other tokens that have markets with $aUSD can be swapped into $ACA and hence also used for gas fees.
Note: if you want to use $DOT as collateral, you can either withdraw from centralized exchanges to the Polkadot network, and use the Acala bridge here or swap $ACA withdrawn directly onto the Acala network for $DOT here.