First of all, let's say that when dealing with chemicals, you MUST follow safety rules, whether you like it or not. It is not acceptable to not take safety measures. Just because you haven't had any accidents with a certain chemical once, doesn't make you tough or lucky.
As important as the internal maintenance of the unit is, the external maintenance is equally important. We cannot talk about internal filters and have the external unit dirty. It's contradictory, because it takes in air and releases it inside. So, only the inside should be clean but not the outside? - You might say that maintenance technicians do this during the "hot season" when everyone is rushing for maintenance.
Now, regarding the instructions for use. We are talking about diluting 1/5, which means that in a liter of final liquid, we add 200ml to 400ml of the chemical (depending on how dirty the external unit is) and the remaining 800-600ml is water. You will also need a sprayer with a hose (the ones used for pest control, starting at 7-10 euros from a well-known "blue/yellow" store). Be careful not to spray the power supply. So, we spray the back, the side from the other side, and the inside after removing the filter and the same filter (the round one that covers the propeller). We wait for 10-15 minutes and then rinse thoroughly with a hose and pressure. We also remove the cap under the air conditioner and all the dirt comes out. We wait for 6-8 hours and then we turn it on to make sure it has dried well, even if we accidentally wet the power supply. The whole process assumes that the internal unit is not plugged in - it's not enough for it to be turned off. I don't know if it works miracles, but I do know that it is a gift of health that you give to yourselves. The external appearance is like new. It removes dirt such as exhaust fumes, dust, or pine needles. Cost? Again, for me, it is very cheap since with one liter, you can clean about 4-5 air conditioners once a year. So, if the package contains 4 liters of the chemical, it will last for a good decade... so 12 euros (average price) every 10 years (400ml x 10 times/once a year and our mind is at ease)... expensive? And of course, the same applies more or less to the other chemical for the internal units, with the only difference being that you don't rinse it if you don't want to and that the pressure is manual, not with a cable.
P.S. Finally, it should be noted that I am an amateur and I choose to do it myself. If you can't or are afraid, it is absolutely fair and better to trust a professional, while also asking for the cleaning of the external unit in spring and not in the middle of summer.