An instructive text is a text that instructs or tells you how to do something

A recipe wants to instruct you how to cook something.
A leaflet with a piece of furniture wants to tell you how to put it together or take care of it.

Instructive texts:

are written as though the reader is being spoken to -
(although the word 'you' is not usually used)
language is direct and unnecessary words are left out
often use 'must' and 'must not'
sometimes use diagrams or pictures to help understanding

Examples:
Put all ingredients into bowl together. Whisk until fully mixed.

Go to the end of the road and turn left past the pub on the corner. Keep walking until you come to a park and then turn right into Hawker Street.