there are some books on that

and how to draw them

Okay here we go;

First here is a site that teaches knot tying
http://www.earlham.edu/~peters/knotlink.htm

Here are some celtic knot patterns
http://www.abbott.demon.co.uk/knots.html

And here is a Blog for celtic knot tying with useful links especially for celtic knot tying.
http://www.ljseek.com/How-To-Tie-Celtic-Knots_s4Zp1.html

Good Luck and don’t get sore fingers!

It is very hard to tie celtic knots in real string, as the knots have a lot of corners and open spaces that string will not keep.
You can tie the knots pinned down on a board and stiffen before you take it of, but that is not a guaranty that the knot stays in shape. And hemp is known to change colour dramatically when wet, and when treated with glue.

The only knottyer I know who ties celtic designs with success is using silver and gold, it is Loren Damewood, also known as Goldenknots, an example:
http://www.golden-knots.com/bayarearenfair07/amber01.jpg
Main site, see galleries for more of his celtic work:
http://www.golden-knots.com/

There is a book on the market which promises to use Celtic knots in jewelry, but they use basic Turks Head knots:
http://knopen.ismijnhobby.nl/tables/turkshead_table.html
and other woven knots:
http://knopen.ismijnhobby.nl/tables/diamont-4_table.html
instead of beads.

Here is a link to a site where you can buy the book:
http://www.amazon.co.uk/s/ref=nb_ss_w_h_/026-5760561-4609258?initialSearch=1&url=search-alias%3Daps&field-keywords=celtic+knots+for+jewelry

Sorry not to be able to help you further, but you have picked a hard one this time.
Feel free to contact me for more info or hints.