Create an assignment as usual, but tick the Extra Credit check box near the top of the new assignment display, and then enter the Extra Credit Percent into the adjacent box. The extra credit percent is the percent that will be added to the student's grade if they get 100% on the assignment.
Make sure that you enter Points for the assignment. Scale Factor and Total Points are ignored unless you subsequently make the assignment a non-extra-credit assignment.
Note that Extra Credit assignments are hidden from students who do not earn extra credit. Also note that extra credit is added to the overall grade and not to term or category grades. When describing extra credit grades to students, it is correct to say "This is worth x% extra on your overall grade."