Forms should be clear and easy to use. You must consider the way the form will be used and the language that you use to explain the steps in completing the form.
The accessibility of the form must be taken into consideration at a technical level and you should follow the WCAG techniques for creating accessible forms. This article from Web AIM has some great advice on how to do this.
This information from the FACS Digital Accessibility standard by the FACS Digital team is licensed under CC BY-NC 3.0