Ancient gates
Look for an ancient gate in an Overworld cave. Carved stone, blue lanterns and a glowing opening mark the entrance. Walk through to enter Moria; the gate is already active.
The return gate takes you back to the cave you came from. Other gates in Moria can lead to different Overworld locations. Their frames cannot be crafted or broken in Survival mode.
You can also find gates in caves added by other mods.
Nether travel
Build an obsidian portal in Moria and light it as usual. It leads to the Nether; use that portal again to return to Moria.
Waystones
With Waystones installed, a waystone stands in each village square and in the passage leading to a throne room. Activate them as you explore. The throne waystone lets you return after death through the usual Waystones travel options.
Finding a gate on a server
If you cannot find an entrance, ask an administrator to use /moria survey 0, followed by /moria status and /moria locate.
For areas explored before installing Moria, use /moria retrofit. The command guide explains these searches and how to choose their range.