Self Tie cravats are easy to tie. Like a four in hand knot. Here is a video: http://www.youtube.com/watch?v=2-uQRzfndxg
(they can also be tied in a few more unusual knots, if you would like to know I can post pictures later)
Self tie cravats have no adjustment for size so you will not see a slide bar at the back of the shirt. However they are never big enough! So when you have a person with a collar size over 16 1/2 " they do not tie correctly. If you are to make them correctly you will add 2" onto the length of a cravat for every 1/2" collar size. They also need to be silk as polyester fabrics do not grip or scrunch up, the way a cravat should.
Ready Tied cravats are better as one size fits all and they all look the same. If you buy them with a floating neck band you can spin the band around so the clasp is hidden under the shirt collar. You will however see a slide bar at some point around the collar of the shirt.
In the end it does not make a huge difference. I use both styles side by side and most people would not know the difference. http://www.hitched.co.uk/mens-wedding-suits/jack-bunneys/ I bet you can't tell which ones are ready tied!!
Good Luck !
Marc