There's no real trick to it, Steve. You need a good pair of tweezers, a small, SHARP pair of scissors, and a small bottle of white glue. (Some folks prefer cyano adhesive for this job, but I like white glue because it's easy to reverse in case you make a mistake.)
It also helps a great deal to work with good rigging line. Forget the stuff sold in sewing stores. Buy your line from a reputable ship model supplier, such as Calder/Jotika, Syren Ship Model Company, Bluejacket, or Model Expo. (I'm sure there are other good ones in Britain.)
And make sure the line is the right size. A good sailing ship model requires lots of different sizes of line. For most purposes, three or four probably will be enough.
About ninety percent of the time you can get along with one type of knot: the one Boy Scouts call the square knot and sailors call the reef knot. You probably already know how to tie it, but here's a tutorial: http://www.animatedknots.com/reef/ .
One other knot comes in handy for one purpose: tying ratlines to shrouds. That knot is the clove hitch - one of the simplest of knots to tie: http://www.animatedknots.com/cloveend/ .
Pull the knot as taut as you can, and apply a tiny bit of the glue (using the end of a toothpick or something similar). Let the glue dry, then snip off the end with the scissors.
It does take practice. Rigging a sailing ship model is not something that can be done really well the first time. But keep at it and your work will get better.
Hope that helps a little. Good luck.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.