To measure the level in your tank, first make sure you have a suitable dipstick (this could be a clean garden cane or a broom handle) and a tape measure.
Take the cap off of the tank where the fuel goes in and put the dipstick right down to the bottom of the tank, making sure the dipstick touches the bottom.
When you withdraw the dipstick, the oil will leave a mark on the stick showing the level of oil in the tank. Take a tape measure and measure the distance from the bottom of the stick to the top of the oil level mark and record the measurement in mm.
Then put the cap back and submit your reading online.
When submitting your reading, it's OK to do it after you took the reading but if you do, please enter the date the reading was taken, rather than the date you are submitting it.