Since this type of window is usually in height, a scale will probably be necessary to install it, and then do the maintenance. Make sure you feel comfortable with heights and to remove any source of potential danger. Condensation from atmospheric conditions inside and outside the house; this normal problem must be contained within.
Regarding maintenance, it is better to use a cleanser nonabrasive glass or mild soap and a soft brush and remove the cleaning solution immediately with a dry cloth or squeegee.
Do not use blade to clean the glass, as it may cause permanent damage. If it is really necessary, use a small razor blade on a very small surface and scratch always in the same direction.
As for the internal frame, a wet cloth is generally enough to clean it. However, make sure to paint or to varnish the frame quickly after the installation if it is a raw wooden frame.
Finally, on the outside, remove any debris around the skylight before cleaning the frame and the glass with a slightly soapy water and rinse well. Take the opportunity to make sure there is no damage around the skylight. If this is the case, make the repairs as quickly as possible.