I recommend 3/8" to 1/4" braided nylon rope, avilable by the foot from most Home Depots. Don't get the prebagged stuff, as it generally uses loose fibers for fill.
For a basic wrist tie, have the victim hold her hands about four to six inches apart, and wrap the rope around both of them. Now add cinching loops by wrapping that middle section in alternating directions. Finish with a square knot or a sheet bend. It may take a bit of experimenting to get it tight enough that she can't slip out easily, while still loose enough not to cut off circulation. You _may_ have to start over from scratch. Be patient. Make sure you can fit two fingers under the ropes.
Knives are great, but you won't have much room for "follow through", so use a serrated edge, or a plainedge sharpened coarsely. Alternately, use paramedic scissors. But whatever you do, have an emergency cutting tool on hand. Knots _do_ jam.
If you're ready for some more advanced techniques, you can start with
Rainbowrope's rope bondage tutorials. (The tutorials contain some _minor_ nudity--don't follow the link if you don't want to

)
Just keep it safe, sane, and consensual
