Engine oil can be poured by opening the lid of the engine oil tank located in the engine. First empty the old existing oil and then pour the new oil upto the required level.
Not know what year of Harley you have or what model it is i can't say exactly how it is done. A lot of people recommend filling the primary by using a funnel and some tubing to fill it through the chain inspection hole. Just fill it up to the bottom of the clutch basket while level.
of course it depends on year but, in general the owners manual says to remove the derby cover and fill with 32oz of Harley approved fluid. It is much easier to remove the chain inspection cover and fill through there. do not overfill!
Stand the motorcycle up straight with the help of an assistant or centerstand. Refill primary oil through the open derby cover until the oil reaches the lip of the opening.