We would recommend that the tubers are planted up as soon as possible into individual 6-8” pots of just moist potting compost (with their crown at or just above compost level) then placed in a warm, light position such as a greenhouse or conservatory.
Keep the compost moist but not wet, increasing the amount given as growth develops whilst always avoiding wetting the foliage. The plants will be well advanced for planting outside when the danger of frost has past (usually in late may) at which time they can be carefully knocked out of their pots and planted approx 20” apart in a sunny spot with well-drained, fertile soil.
If it is not possible to start the tubers into growth early, as outlined above, remove them from their bags upon arrival and store frost free for planting direct into the garden in early May. In either case the tips of the growing plants may be ‘pinched out’ in late June to encourage bushy development.