1) Make a generous loop in the rope you will use, size according to purpose.
2) Use electrical tape to create two bands around the rope to stop it unravelling more than you require.
3) It’s all about starting right, if it looks untidy and not uniform it’s wrong. No two strands should be passing under the same twist.
4) The strands can be gradually thinned so the splice tapers out, but as long as the spice has three passes under and over it will never come undone under any load.
5) It’s common to use cored rope rather than basic three ply rope for mooring lines these days, but three ply can be shortened and re spiced much more easily than cored rope and if, as is common with mooring lines, they get damaged most, of the rope length is still usable.