Alternative text is the first and it takes seconds per post. Most platforms provide a field for it, frequently hidden behind an advanced settings option that nobody opens. Describing what the image shows, and why it is there, means somebody using a screen reader receives the same information as everybody else rather than an announcement that an image exists.
Write it for the purpose rather than describing every detail. An image illustrating a completed job needs a description of what was done. A photograph accompanying a quote needs to convey what the picture adds. If the image is purely decorative and the post makes sense without it, a brief description or none is appropriate.
Captions on video matter more than most people expect, because the majority of social video is watched without sound. Automatic captions are available on most platforms and they need reviewing, since they reliably mangle names, technical terms, and anything specific to your industry. Reviewing them takes a minute and prevents the version where your business name appears as something unintended.
Text inside images is the third and it is the most common failure. A graphic containing your offer, your hours, or an announcement is invisible to anybody using a screen reader and unsearchable by anything else. Putting the same text in the post itself, alongside the image, solves it entirely and costs nothing.
Watch the specific practices that cause problems without anybody intending them. Long strings of hashtags are read aloud individually unless written in a form where each word is capitalised, which lets a screen reader separate them. Decorative characters and stylised unicode fonts are read as their underlying character names, producing something unintelligible. Emoji are announced by description, which is fine in moderation and tiresome in quantity.
Contrast applies to graphics as well as websites, and it is where designed social content frequently fails. Text over a photograph, light text on a pale background, or a colour combination that looked good on a bright screen can be unreadable in daylight on a phone, which is the condition most of your audience is in.
Consider that these practices help beyond accessibility, which is the argument that usually persuades. Alternative text is read by search systems. Captions increase watch time substantially. Text in the post rather than in an image is searchable and quotable. Each of these is a marketing benefit that happens to also be the accessible choice.
Then build it into how you post rather than treating it as a review step, since retrofitting a year of content is a project nobody completes. Written when the post is created, it costs a minute. Added later, it costs an afternoon and usually does not happen.
Check how your posts read when spoken aloud, which is the fastest way to catch the problems that are invisible on screen. Reading a caption out including the hashtags and the emoji reveals immediately whether somebody using a screen reader would follow it.
Fix the templates you reuse rather than each post individually, since a graphic template with poor contrast reproduces the problem every time it is used. One correction there addresses everything produced afterward.
Describe charts and data images properly rather than naming them, since a screen reader announcing that an image is a graph conveys nothing. Stating the actual finding in the description or in the post itself is what makes the information available.