An instruction field shows a read-only block of text in the design generator. It does not collect any input or connect to the design; it is purely there to guide the people filling in the form, for example to introduce a section or give a tip.
Add it from the Inputs panel: click Instruction, then type your text in the Instruction box. Reorder it like any other field to place the guidance where it is needed.
For help attached to a single field rather than a standalone block, use that field's Description instead.