The options under Available to Run define how the workflow can be initiated in addition to the triggers defined for the workflow.
When created, classic workflows default to run in the background. The Run this workflow in the background setting is grayed out. You cannot change from background to real time by editing the field. You must use the Convert to a real-time workflow button in the toolbar.
If the As an on-demand process setting is checked, then users can select and run this workflow in a model-driven app form by clicking on the Run Flow button in the command bar. The on-demand process setting also allows the workflow to be called as a global workflow or a stage transition workflow in a business process flow.
If the As a child process setting is checked, then this workflow can be called from another workflow or action.
Once you have set your triggers, you can add your steps to the process.