To obtain extra map for memorial, one should do the following:
Prepare two sets of Telekinetic Grab runes (2 laws and 2 airs, also requires level 33 Magic) to obtain two maps from Wormbrain. Players should kill Wormbrain once and get hold of the map for several minutes, until they can attack him again (hopping worlds resets the timer), but firstly drop their piece and grab the second, or he will not be attackable.
Players don't need double items for obtaining other double map parts, just drop the first and grab the second, and then pick the first one as well.
Combining the map is an important part. Never place a map in a bank when combining another map. Instead, once all the six pieces are in the inventory, drop three of them and combine the others. Then, pick up the three pieces and drop the map. Combine and pick up the combined map.
Before giving one of the maps to Ned, it isn't safe yet. Just drop a map and bring another to Ned. Then, pick up the dropped one and don't talk to Ned again. Bank the map or keep it in the inventory (be careful not to die with the map) and climb aboard the Lady Lumbridge, talk to Ned on the ship, and go fight Elvarg.
If a player manages to keep the map and take it to the Wise Old Man, he will offer to buy it for the price of 3 coins.