Step 1: Pick your writing situation.
Remember that you want a relevant, interesting, real-life writing topic. Think about the audiences and purposes that your students might have for real-life writing experiences.
To connect with the reading assignment example, I would imagine that my students might send an e-mail to a friend to make plans to take a boat trip.
Step 3: Include Grammar Structures
Decide if you want or need to include grammatical structures to aid students as they write. This decision will depend on the purpose of the writing. If it is a practice activity early in the unit, you may want to provide the extra scaffolding. If it is a formative or summative assessment, then you probably don't want to include grammartical structures.
I plan to use this writing assignment early in a unit of study, so I will include some new grammatical structures to get students started. In this case, I plan to teach future tenses in this unit, so I'll include a few examples of sentence starters in both future tenses. That will also help with our class work on the difference between the two tenses.
Step 2: Decide the details.
Decide the details that you want to require of your students. Do you want to specify a length, audience, vocabulary usage, etc.? Then type that up as your directions.
For this assignment, I am not including a length, but I am going to elaborate on what topics I want the students to include. I want them to pick a trip to take and invite a friend via e-mail. In their invitation, they need to include details of the trip (time, cost, etc.)
Step 4: Pull it all together!
Type up your work to use in class.