What to Write
Any notes that will help you DYJ on the day (Do Your Job). A good start is: Start Time, Distance At Key Climbs or Features e.g. Gravel, Finish Distance or Rival Bib Numbers.
This can also be applied to training by having your power and heart rate zones on your stem. This info may also be useful for metering efforts e.g. in a solo breakaway or on a long climb. If you have a complicated training session you could make notes to help you remember the various efforts and intensities.
Why Do I Need It?
By putting the start time is at the top of the list, you have no reason to be late and therefore be in a bad position from the get go. Also, by knowing exactly where the finish is, you can count down the distance to go on your computer, and make sure you are well positioned for your sprint. If you know where key features are, you can be positioned ideally ahead of time. Positioning can take time. You don’t want to find yourself off the back just as the bunch hits the gravel section. Especially for stage races, its useful to now exactly who you have to mark, and if you have their number in front of you you don’t have to remember exactly what they look like. Just check the numbers.
How To Stick It To Your Bike
Keep a roll of masking tape in your race bag along with a fine pointed Sharpie. On the night before, go through the race info and write the important stuff on a strip of tape and add it to your stem. Use shorthand e.g. S 10.2 means sprint at 10.2km. By keeping the pen and tape in your race bag, you will have it handy if you need to write something on the fly at the race e.g. if new info comes up.
Snag one of those kitchen label makers that prints off a sticky label. You’ll have clear notes that won’t come off.