During a handfasting ceremony the couple hold hands and the cords are used to fasten their hands together just before, just after or during their vows. The wrapping of the cord symbolises infinity, the handfasting knot that is tied is symbolic of their oneness, it shows unity as they become bound to each other.
Tying the cord can be done 'Celtic style' where the couple hold hands and the cord is wrapped three times around or it can be done 'Infinity style' where the cord is wrapped across the hands, one side loops around one partners hand and the other side loops around the others, it then meets back on the top and is tied, thus the cords form an infinity symbol.
There seem to be various different schools of thought about what length hand fasting cords should be, ranging from 1 yard, 3 yards, 3 feet, 6 feet, 9 feet so I guess it is a case of working with what suits you as long as it is long enough to wrap around both wrists and enough to tie a knot.
I have also seen cords that are made up from 3, 6, 9, 12 and 13 separate strands. Some are made to match the colour of the hand fasting outfits, sometimes colours are picked that have specific meaning for the couple, some are white & silver, some white, red & black and the traditional 13 are all different colours, each with a specific meaning.
Orange: encouragement, attraction, kindness, plenty
Yellow: charm, confidence, joy, balance
Green: finances, fertility, charity, prosperity, health
Blue: tranquility, patience, devotion, sincerity
Purple: Power, piety, sanctity, sentimentality
Black: strength, wisdom, vision, success
White: purity, concentration, meditation, peace
Gray: neutrality, canceling, balance
Pink: unity, honor, truth, romance, happiness
Brown: earth, grounding, talent, telepathy, home
Silver: treasure, values, creativity, inspiration
Gold: energy, wealth, intelligence, longevity
The ribbons/cords can be knotted together in the centre with a Josephine knot or the middle section can be plaited leaving a length un-braided at either end.
You can also add charms and beads either knotted onto the individual cords or tied onto the ends.
There are plenty of websites on the net that give instructions on how to make handfasting cords, just Google 'how to make handfasting cords' and you will get lots of different versions.