Do know that Jens left Opdal for Seattle April 11, 1900. Used a ship on the Allan Line from England to America. How do I find the ship he left England and what port he arrived in America.
In an earlier posting about this man you stated that you had a document which listed his port of entry to the US as New York (although the original year listed was later seen as questionable). Have you already checked the Ellis Island website? Checked for records of the other ports of arrival for that line given you as likely in that first thread? Concentrate on records of passenger arrival in the US and/or Canada.
In the Articles posted on this website you will find several excellent how-to's for exactly the subject you request.