Go to "Processes" tab (1) and click "New Process" (2) button:

Process creation wizard will be shown in popup (3). Popuplate Process name (mandatory field), description (optional),  select manager if needed, and specify process admin(s) (by default current user will be set automatically):

Click "Next" button (4) to show next step of wizard (5). Select data source object (6) and check checkbox (7) if you want process to be allowed to start only once:

If you check checkbox (7), the "Source record" lookup field will be shown (8). Here you can pre-select the record to start process at, and when you start this process, record will be set automatically, so, nobody will control which record to select. If you do not populate this field, the process will be started as usual (you will be asked for a record), but only once.

Click "Next" to proceed to the following wizard step (9). Select method to start the process (10) and check "Allow duplicates between versions" checkbox (11) if needed:

Set up process notifications at the next wizard step (12):

At the last wizard step (13) you can review your data and save the process:

The process builder will be opened (14). You can edit some process settings by clicking the proper menu item (15) in the settings popup (16):

The newly created Process must be activated to be usable. No starting of Process instances available without activation.

To activate the process open "Actions" menu in process builder and click "Activate" button (17; for already activated process this button will become "Deactivate").

Activated process is indicated with green dot at the proper process tab on the builder (18). Inactive process has the red dot (19):

Also system will offer a Process activation  for the current process (20) only, or with subflows, if any (21) when user clicks "Save" button: