You'll need to set your availability for your online booking in time slots of your choice before your customers can book you online.
The most popular time slot is 2 hours, however it can be customized to your business. Remember: the slot that you are setting is the window of time in which you'll arrive for the job, not how long the job will take to complete.
To set your online booking windows, go to your account settings by clicking the user icon in the top right, then account settings.
Then choose 'Business Hours.'
This will take you to the page where you can set your custom windows. To add a new slot, just hit '+booking window' toward the bottom right of the card.
Choose your time slot and then select the days and time you want that slot bookable online. Do this for each individual time slot you'd like available and you're done!