The follwing example uses the aria-describedby attribute on an input element to reference the descriptive paragraph ("Please provide...") qualifying the input needed. It also uses the label element to provide a visible label (example adapted from Steve Faulkner's HTML5 Accessibility Chops: form control labeling).
In all the tests documented here, there was no difference in output when the descriptive paragraph referenced by aria-describedby was given a tabindex attribute (tabindex="-1") (see variant of this page with tabindex=-1 on the element referenced). However, according to Steve Faulkner, tabindex="-1" should be provided on non-interactive elements referenced, to ensure support for some browser / screen reader combinations such as IE9 and JAWS 11 (not tested here).
Please provide both first name and surname here.
Results of screen reader tests for aria-describedby to add a description to a text input field
iOS 7.1 (on iPhone 5c), VoiceOver
Swiping: Reads first the content of aria-label then element type "Your name - text field. Does not read element referenced by aria-describedby
Touch-explore: Same behaviour as with swiping
Android 4.4.2 (on Nexus 4), Firefox 28.0.1, TalkBack
Swiping: Reads first the content of aria-label then the sentence referenced by aria-describedby: "Your name - Please provide both first name and surname here". No output of element type (text field).