Appeal to their self interest.
To see that, they need proof that digital marketing generates money. Most small business are very short term, cash flow oriented.
So (1), get your digital dashboard in place (I use Agency Analytics but pick what you like)
(2), show the client his/her sales funnel (it is useful if the dashboard allows client accounts too so they can get the info by themselves when they want to), teach them how it works
(3), by now the client should grasp the rationale for improved traffic building (organic or sea), behavioral monitoring and tweaking, resulting in higher conversions and transaction sizes
(4), discuss with the client ways to improve the sales funnel’s performance, suggest paid and cashless options such as AdWords and other sea, linkbuilding, content production, social media, offline traffic building, et cetera
(5) help the client to implement your recommendations
This how I do it for my clients on a daily basis.
SME clients love any clear, direct, logical, comprehensible approach to anything that brings in the bacon. If your invoices and terms are fair you can expect to do some brisk business.