Linux VPS servers

Log in to your WHM panel. Open up the 'Edit DNS Zone' page, and select the domain you'd like to add this to. Select 'Edit'.

At the bottom of the page, there's an area that says:
'Add New Entries Below this Line'

Add this line in the first box:

yourdomain.tld.

The remaining boxes should say:

14400 TXT "v=spf1 a mx ~all"