Dahlias are not frost hardy and so need to be lifted for the winter months.
Dahlias produce a swollen root system and it is these roots that need to be protected.
Your first job will be to cut back the aerial part of the plant right down to the base of the plant.
Use a garden fork to dig up the whole root system of your Dahlia, here you will find several swollen roots all connected to the base of the plant. You need to keep it all together but you can brush off the old compost from around the roots. If any of the swollen roots come away from the roots system you can discard these as they will not come to anything on their own. However, in the spring when they begin to show signs of new shoots and roots of their own then they can be divided.
For now, your root system needs to be placed in a box or a large enough pot and covered with compost, this compost will prevent the swollen roots from drying out over winter. They now need a place that is frost free for the winter months to survive.
Remember, having got them through the winter, don’t be too keen to plant them out in the spring. Wait until the last of the frosts have passed.