Just dig up the roots. Be careful not to disturb the root system too much - it's easy to leave behind bits and pieces, and with comfrey, each tiny bit left behind will become a new plant - this is how comfrey can easily become a weed. It can be hard work, because comfrey has a very long tap root which might go down into the ground a metre or more.

Then dry as follows (instructions apply to other roots, too):
Most roots are harvested in autumn for drying. Scrub clean, removing small rootlets. Peel if necessary. Chop into small chunks or tiny pieces, depending on your need. Lay them flat, not touching each other in flat trays or baskets. Find a cool, dry, dark place for them to dry, usually 2-3 weeks. When they are totally dry, they will feel dry but be spongy to the touch. They should not be rock hard. Put them in jars, label them and place them in a cool, dry cupboard somewhere until you need them. A paper towel or napkin with dried milk powder folded and place in the bottom of the jar will help to absorb any extra moisture (replace every few months). Paper or burlap bags are great for keeping roots in; place the bags in a box or keep in a cupboard to keep safe.

thanks daisyd. appreciate it.

Many thanks for your instructions regarding how too dry comfrey roots. Last a few years I have plenty comfrey plant in my garden. I am happy to give anyone who lives in London, UK. A few questions?
May I dry them in a conventional oven?
How to use the dry root?
Is it safe to use internally?
Many thanks again
Best regards
Aysegul

You can dry the roots in the oven. The usual method is to chop them into equal-sized pieces, spread them evenly onto an oven tray, then turn on the oven to the lowest possible temperature. Keep the oven door ajar while drying the roots, and check the roots often, stirring them around to ensure even drying. It will take several hours.

Pounded root bound round the area when a bone is broken will set like a plaster case when dry.

Comfrey is no longer recommended for internal use, but was once used to treat irritable bowel syndrome, ulcers, colitis and a range of respiratory conditions such as bronchitis and pleurisy. The wisdom of using comfrey internally is widely debated. In some countries, it is illegal to prescribe it for internal use. The highest concentration of potentially harmful alkaloids, which may cause cancer or liver damage or even death, is in the roots but some experts consider the leaves safe to use internally, especially if they have been dried. Its healing powers are so strong that wounds may heal on the surface, sealing in any infection deeper down. Avoid long-term use.

Usual Dosage: Cover 3 heaped teaspoons fresh or dried rootstock with 1 cup water. Let stand 10 hours, strain. Bring the rootstock to the boil in 1/2 cup water, then strain. Mix this with the cold extract and drink a mouthful at a time over the course of the day. Decoction: Put 1-3 teaspoons of the dried herb in 1 cup water, bring to the boil and simmer for 10-1 minutes. Take 1/2-1 cup every hour until condition is gone or 1 cup 3 times per day.

Comfrey root has been shown to contain ten times the amount of liver-damaging alkaloids present in the leaves (this applies to Symphytum officinale, apparently), so using the roots is particularly hazardous. As to external use, good evidence for healing of wounds is very limited, and many herbalists as well as the Natural Medicines Comprehensive Database urge that comfrey preparations only be applied to unbroken skin, to minimize the absorption of the harmful alkaloids.

Personally I think comfrey is one herb to enjoy only in the garden and to appreciate for its historical appeal, while recognizing that the old-timers who enthusiastically recommended it weren't so good at evaluating either its efficacy or toxicity.

Those who believe the contemporary take on comfrey use buy into all the lies we've been fed for decades due to money & control issues. If you don't know enough about a particular plant's energetics, which is much more important by far than knowing about it's biochemistry, find someone who you trust that does. Otherwise it's just fear & greed that continues to rule the day. Comfrey is an incredibly useful herb in the right hands (& heart & mind).