TIP: Instead of a latch needle, make a 24" loop puller from #58 wire by bending wire and pinching at the midpoint.
With end of line towards you, insert loop puller tool about 12" from tag end, facing the spool. Push tool about 12" inside main line, going AWAY from tag end, and exit.
At point where tool exits, form a small loop from main line and insert tip of loop into tool. Pull this loop back inside main line. Main line will literally turn inside out. Inserting the smallest tip of loop possible will make this step easier.
Pull loop all the way through and exit. Remove tool and adjust loop to size. Make loop large enough to accommodate size of topshot spool, or wind-on leader coil. Tag end will be hanging along side of main line.
Insert loop puller into main line at point where tag ends. Push tool up towards loop splice and exit at base of splice. Insert tag end into tool and pull all the way back through main line. Exit and remove tool.
Pull ends of both lines to close splice in center. Trim loose ends, and smooth out to bury them inside main line. Finished end loop is 100% strong and will not pull out. Confirm by pulling one side of loop out to the lockpoint.