Alternative text is the quickest because it requires no technical change, only writing. Every image conveying information needs a short description of what it shows and why it is there. Images that are purely decorative should have an empty alternative rather than a description, so screen readers skip them rather than announcing a filename.
Write the description for the purpose rather than the picture. A photograph of your team on an about page is described differently from the same photograph used as a link to a services page, because in the second case what matters is where it goes. The test is what somebody would need to know if the image did not load.
Contrast is the second and it is checkable with a free tool in minutes. Text needs a measurable ratio against its background, and the usual failures are grey text on white, light text on a photograph, and anything using a mid tone colour that looked good in a palette. Placeholder text in form fields is a frequent offender that nobody thinks to check.
The keyboard test is the one that finds structural problems rather than cosmetic ones. Tab through your own site and watch where the focus goes. If you cannot see which element is selected, the focus indicator has been removed, which is a common styling decision that makes the site unusable without a mouse. If you cannot reach the navigation, a menu, or a form, that is a genuine barrier.
Form labels are the fourth quick win and the one that affects conversion as well as accessibility. Every field needs a label associated with it programmatically rather than only visually, and placeholder text is not a label because it disappears when somebody starts typing. This is one attribute per field.
Add a skip link and a main landmark if the site lacks them, which together let somebody using a screen reader reach the content without traversing the navigation on every page. Both are a few lines and neither changes anything visible.
Caption your video, since a large share of video is watched without sound regardless of ability. Automatic captions from the platform are a reasonable starting point and they need reviewing, because they reliably mangle names, technical terms, and anything specific to your industry.
Then check headings are in order, which is invisible and structurally important. Screen reader users navigate by heading, and a page whose headings jump levels or exist only for styling is considerably harder to move through. Fixing that is usually a matter of using the right tag rather than changing how anything looks.
Fix the template rather than the pages wherever the problem is structural, since one change to a shared header, footer, or form component corrects every page at once. Working page by page on something that lives in a template is the slowest possible route to the same outcome.
Check it again after any redesign, since accessibility decisions are frequently undone by a change made for visual reasons. Contrast in particular tends to degrade whenever somebody adjusts a colour palette without checking the ratios.
Ask somebody who uses assistive technology to try your site if you can, since fifteen minutes of that is worth considerably more than any automated report. Automated tools catch roughly a third of real problems, and the rest are only visible to somebody actually navigating with them.