Here's the map showing the reading order:


And here are the issues:
Uncanny X-Men no.210
Uncanny X-Men no.211
Uncanny X-Men no.212
Uncanny X-Men no.213

Uncanny X-Men no.210:
Rogue goes on a shopping trip. Magneto accepts an invitation from the Hellfire Club to taks about joining their ranks. Dazzler is haunted by Malice. ....and the Marauders form.

and no.211:
The Marauders start "cleaning" the tunnels beneath New York. The X-Men try to save as many people as possible but they get clobbered.

Uncanny X-Men no.212:
The X-Men are still healing from the beating they gor from the Marauders, Wolverine is still roaming the tunnels where he gets in a fight with Sabertooth and Callisto confronts Storm with her respocibilities as the leader of the Morlocks.

Uncanny X-Men no.213:
The X-Men are all back at the mansion when Sabertooth descides to pay them a visit. Betsy stops him long enough for Wolverine to take over and defeat Sabretooth.
X-Factor no.9
X-Factor no.10
X-Factor no.11

X-Factor no.9:
Freedom Force is after Rusty and Skids but just when they have them cornered X-Factor intervenes. The two youngers flee into the sewers where Freedom Force and X-Factor go all out. Then Destiny sees in the future that anyone left in the sewers will die so Freedom Force depart.

X-Factor no.10:
After it confrontation with Freedom Force, X-Factor is returning to its HQ thru the tunnels beneath New York. Then they stuble onto the Marauders killing the Morlocks. X-Force descide to intervene, but they get clobbered. Angel meanwhile is chaught by the Marauders while attempting to rescue Artie.

X-Factor no.11:
X-Factor and a group of left over Morlocks escape the tunnels. Once out of the tunnels the group of Morlocks gets into trouble with a gang and after that with the police. X-Factor is forced to take out the Morlocks.

Thor no.373
Thor no.374


Thor finds himself in the tunnels underneath New York when he hears the scream of Angel who is being tortured by the Marauders. Thor takes on the Marauders and defeats them. Then he brings the wounded Angel and scared Artie back to X-Factor.
(These scans were not scanned by myself. I got them from a newsgroup and don't remember who posted them.
If anyone wants to claim the credit let me know, I'll add your name)
Power Pack no.27

The Power Pack go into the tunnels under New York to save Leech and Caliban from the Marauders. It takes some help from Wolverine but they get the job done.
(I got this issue from Judius X-Men Underground)
New Mutants no.46

The X-Men return to the mansion and the New Mutamts help to set up a hospital for all the wounded Morlocks.