For your first question, ignore the first two zeroes. The pointer would be 50 15 38 08. For the second question, offsets are backwards in the hex. As i said before, ($ABCDEF becomes EF CD AB 08), so 3F089C would be 9C 08 3F 08
Assuming you have a hex editor: Open up your Rom in it. If you consult the pointers in the spoiler on that tutorial (Assuming you have Firered BPRE and are editing the map of Kanto), you need the pointer $3F089C. Take the offset of your tilemap, and turn it into a pointer ($ABCDEF becomes EF CD AB 08). Using the hex editor, search for "9C 08 3F 08". Whenever you find that, replace it with the pointer of your tilemap.