As this is a bare root plant, soak the roots for at least 2 hours before planting, after they arrive. Plant as soon as you can. If the ground is frozen, place into a temporary pot that's filled with decent compost- once conditions have improved, move the blackberry plant to the ground.
Keep the plant well-watered during dry weather.̴Ì?
As the Blackberry Merton plant will flower on one-year-old wood, you will need to separate the new canes from the fruiting ones (old canes). After it has been harvested, cut down the fruited canes to ground level, and tie in the new canes that have just sprouted (these will be the new fruiting canes the following year).
Repeat this every year and you'll̴Ì? be gifted loads of delicious berries every year!