Accessibility of forms has a lot to do with labels and names. Use visual labels as much as possible. Make sure the labels are connected to the right fields. If you can't use a visual label, at least consider providing your users with visually hidden labels.
Remember that a placeholder is not a label. It should provide an example of how the field should be filled. But note that the placeholder is not supposed to be actual field instruction either.
If forms are created carelessly, there is a big chance that they will be unusable for some people. So always remember to pay extra attention to forms and test their functionality with screen readers.
Use proper field types, e.g. don't use a text field for a phone number.
Remember to always use <fieldset> and <legend> when necessary. They are very important with checklists and radio buttons. Don't use the elements to lay out the form content.
Use autocomplete only if it's supposed to be used in each field. But do offer autocomplete when you can, it helps all users.
Provide instructions, which are linked to the correct fields. You can link the instructions to the input field with aria-describedby.
If your form is multiple pages, the user needs to be able to confirm everything before sending the form. Especially if money is involved.
CAPTCHAs can often create problems for many people. If CAPTCHA is used, make sure that text alternatives that identify and describe the purpose of the non-text content are provided, and that alternative forms of CAPTCHA using output modes for different types of sensory perception are provided to accommodate different disabilities.