It's not usually something you plan, but it will happen. You will need to miss school for one reason or another. And so many times it comes down to the afternoon or evening before and you think, "I forgot to do the sub plans!" Well, don't panic. Following the Girl Scouts' lead of being prepared makes for a better day for everyone involved.
Much of the information a sub would need can be gathered well ahead of time. Items like your general instructions of classroom rules, behavior issues, helpful students, health concerns, handy phone numbers, and instructions for using technology can all be written near the beginning of the year. I know many teachers have a standing sub folder, file, bin, or other container with this information. I would also suggest keeping nurse passes, attendance forms, and other necessary materials in that same spot.
Now, the only thing you need to worry about is the actual day you will be absent. Make sure those lesson plans are as detailed as possible. They should include you schedule, specials rotation, lunch choices, students leaving for pull-outs, etc. And please, please, please don't write, "The students know what to do." Something peculiar happens when a substitute enters the room, and no student "knows what to do." Finally, make sure all copies are made and materials needed for lessons are accessibly (preferably in the sub container I mentioned above).
I have created a handy checklist to download for free. Use it and always BE PREPARED.