Which ever way you decide, it might be obvious this point, but practice, practice and a touch more practice.
It may look easy, but its not, not to begin with. And if you see people on wfp and they seem to do a very quick job, dont assume they have, yet at the same time, dont assume they havent. Its all in the quality of the water being used, the flow and the technique they have mastered.
The final top tip, once you know you can give a quality clean. Be confident in yourself and the service you will give. So do all good window cleaners a favour.......dont under cut just to get a job and dont charge silly cheap prices just to get a job. Long run you really wont do yourself any favours.