Once you've authenticated into the Stacks Wallet with either your Secret Key or hardware device, you'll see an option on the right-hand side of the home screen to "Get started" with Stacking:
Selecting this option will take you to the "Start earning Bitcoin" screen:
On this screen you can see when the next Stacking cycle will begin.
First, determine how much out of your total STX balance you'd like to commit for Stacking:
Then determine how many cycles you would like to commit this indicated amount of STX:
The estimated total duration of those cycles will be shown on the right-hand side of the screen so you know how long your committed STX will remained locked up before getting automatically released again for other purposes.
Enter the Bitcoin address to which you'd like receive rewards from Stacking:
Finally, select "Confirm and lock" to initiate Stacking with the amount, duration and Bitcoin address you've entered.
You will be prompted to enter your password, if you are using the Stacks Wallet with a Secret Key:
Otherwise, if you are using the Stacks Wallet with a hardware device, you will be prompted to confirm the transaction with it:
You should now see a pending transaction on the home screen for "Initiating Stacking". Your indicated amount will become locked on the Stacks blockchain once this transaction is confirmed.
You can follow the Stacking process on the right-hand side of the home screen as well as see how much BTC you have earned as a result: