It's often difficult for substitutes to give students directions exactly as you would.
Not their fault - they're just not you! Leaving your detailed directions as the cover sheet to your assignment ensures every student in every class receives the same information exactly the way you want them to receive it. Leave directions for your sub to read them out loud with the class or really test students and have them read it themselves. There are directions for students to mark their papers to show you they read everything.