Put the initial instructions on how to navigate the form into cell A1 using an input message. Be sure to move the focus to this cell prior to saving the worksheet for the final time. That way, the help text will be seen by sighted users as soon as the worksheet opens, and JAWS or MAGic reads the instructions out loud to their users.
In the Data Validation dialog box:
NOTE: Here is some recommended help text:
Use the list of links or press TAB to move to input areas. Press UP or DOWN ARROW in column A to read through the document.
(The last sentence here depends on how the document is laid out. You may leave this out in more complex forms, since not all field labels or other information may be in the first column.)