Many services for flight planning provide ICAO options. I played around with a couple to see which ones are the most user friendly and caught errors. All of these services are free.
DUATS—This is my favorite ICAO flight planning and filing service. To navigate to the ICAO flight plan area, select “Flight Planning,” “File a Flight Plan,” then “ICAO.” The form closely matches the FAA From 7233-4. Drop down menus make selections easy and clicking a question mark provides an explanation of the item to help completing the form. A drop down menu allows selection of GNSS augmentation (WAAS) and automatically adds the correct notation to the remarks section. Error checking prevents entry of equipment codes without associated remarks. Interestingly, it doesn’t check LPV approach capability with having a GNSS. Otherwise, DUATS is a user-friendly interface for getting used to ICAO FPs.
1800wxbrief—Lockheed Martin’s web version of a flight service station, flight plans can be filed by selecting “Flight Planning & Briefing” from the top navigation bar, then selecting ICAO on the right. The form has dropdowns for certain items containing all ICAO possibilities. Without explanations, this is confusing. For example, flight rule can be VFR, IFR, YFR, or ZFR. The last two are new to most pilots, indicating a composite flight plan (VFR and IFR), but are confusing without an associated explanation.
FLTPLAN—FLTPLAN.com is a great service for charts and creating navigation logs, but falls short on flight planning. Entering aircraft equipment codes is done in a completely different area than the rest of the flight plan form. On the bright side, the website does have funny explanations. My favorite advice they offer is, “if you don’t understand what something is, then you probably don’t have the equipment/capability.”