Aside from using comfrey leaves as a compost activator, a mulch and as a direct feed in the bottom of planting holes/trenches, it can be used to make a 'tea' liquid feed for any veg or fruit plant you grow. So, to make your comfrey tea - first harvest your comfrey. Leaves and stem can both be used. I would recommend the use of gloves for this as comfrey has small hairs on both leaves and stem that can irritate the skin.
There are various vessels you can employ for stewing comfrey tea. I've gone for the small but perfectly formed 2-pint milk container.
I'd definitely recommend a method that has the brew closed in, rather than left open - the smell is absolutely rank.
Stuff said container with as much comfrey as you can possibly cram in.
Fill container with water and replace the lid. Leave for 5-6 weeks to brew - date the bottle to help you keep track. The decomposition of the comfrey in the water may give off gases which could build up in the container. The beauty of the milk container is that the cap usually allows most of these to escape as it's not the tightest fit. If you use a squash or fizzy water bottle then these caps usually fit very tight and so you might need to release any built-up gases every week or so.
Your resulting tea needs diluting down with water before using on plants (1:10 tea to water is usually recommended). Once you've got the hang of this, you may never need to buy liquid plant feed again.