Left-hand column of the Dashboard, “Media”, add/upload the photos you want inserted into the post. When finished, on right-hand side, click “Insert into post”. THEN add the following line with a square bracket at the beginning and the end, that is, a [ the code, and then ]
gallery link=”file” orderby=”title”
above the very first photo’s WordPress code, and you can then delete the code that has all the photos. You will get all the photos you inserted into the post all lined up, and when you click on any one, you will get a slideshow.