Here is a conprehensive answer given by /u/vonadler one month ago. It's one of the best, most in depth answers I've ever read here, so go and check it out.

Here is an excerpt that answers your question, about the organisation and assembling.

A Scandinavian great man decides he wants to go viking. He starts by making sure his longships are in order. During winter, his thralls burn tar and the women make new sails (a long and laborous process). The great man musters his hird, orders them to increase their training and make sure their weapon and armour is in order. He sends out messages to his allies, friends and extended family. Some are great men themselves, and have their own hirds, some are more or less professional vikings and mercenaries. All reply that they'll show up in spring.

The great man also tells his tenants and allied odalbönder of his project, and they agree to join him, providing a few more communaly owned ships to add to the great man's own.

At the winter thing, the great man will probably publically proclaim his intention, sacrifice to the Norse Gods and have wise men and soothsayers interpret the omens. Omens look good, and many of the odalbönder present at the thing promises to show up for the viking.

Come spring, the ships are fitted, the hirds mustered, stores loaded and messages sent out for those that have promised their presence. More longships show up, perhaps somene far away have heard of the expedition and decided it would be fun to join in, arrives with a set of longships and asks to join the viking.

Finally, the great man's own family, allies and hird and dependent tenants and odalbönder have mustered, say 100 hirdmen and 200 odalbönder and perhaps 50 mercenaries/professional vikings (some of which have experience of sieges and have sailed in the waters they intend to go viking in, whose presence does much to increase morale in the host).

Perhaps 100 hirdmen and 1 000 odalbönder have shown up to take part in the viking, and the fleet, now about 150 longships strong, sail off to France or England to plunder. Along the way they may attract more allies as they sail past (a great host often meant easy plunder as the King of France or any of the various Kings in England would pay ransom instead of fighting), but for the sake of this example, we'll say most others are already sailing in viking or trading.

Once they arrive, they sail up a river, guided by the experienced professional vikings and land every now and then to quickly slaughter small bands of militia and peasant levies that try to gather to counter the invading host. It takes the French (we'll assume they are French now) days to gather a levy large enough to combat the vikings, by that time, they are far off.