Take care as σχεδιάζω also means "to plan". I think "design" is more general, and sometimes it is used as it is in English. Because "design" is more than "sketch". There is not such a term that corresponds to "design" in Greek, so many keep it as it is in Greek too. Take a look here, in Greek only unfortunately: https://el.wikipedia.org/wiki/%CE%A3%CF%87%CE%B5%CE%B4%CE%B9%CE%B1%CF%83%CE%BC%CF%8C%CF%82
But if you want to say just "I sketch" you can use one the above mentioned words.
Sketch translates to σχεδιάζω, σκιαγραφώ, ιχνογραφώ. So sketch is a possible translation to σχεδιάζω ^.^
(Keep in mind that σκίτσο = sketch (n.), σκιτσάρω = (I) sketch. They are used, but they are borrowed. ^.^)