Find a sunny spot in the border or position your pot in the sun. Add granular feed to the soil / compost and once you have planted your Dahlia, water regularly to establish. Make sure to direct your can or hose to the bottom of the plant and really soak down to the roots.  If your Dahlia has large flowers you may need to put stakes in the ground, one per stem and tie the stems to the posts to support the weight of the flower.  As flowers fade, cut back stems to a leaf joint, this will encourage the plant to keep producing new buds.  At the end of the season you can leave the Dahlia tuber in the ground if they are planted in a very free draining position and you live in a warmer part of the UK. You will need to cover with at least 6 inches of mulch like bark chippings etc.  You can also dig the tuber up, shake of excess soil and place in a dark, dry & cool place for the winter. Surround them with dry material like sand and re plant in the ground once there is no risk of frost the next spring.