Settings > Path Settings > paste your Ultima folder path then click "Reload paths" ( list should populate with .mul .idx and .uop files )
Setting > Options > Output Path > set to a new local OUTPUT folder ( this is where the mul and uop will output )
Plugins > Manage > turn on MassImportPlugin
Plugins > Manage > turn on UOPpacker
Patching using UOFiddler
the ultima_online_mods folder contains a 00_ART_MODS_MassImport.xml file , ( this xml is a list of item ids and where the modded art files are located ) , edit the filepaths in the XML with "replace all" to match your own , or create a custom XML file using the 00_mod_selector.py tool
Plugins > MassImport > Load XML > select the xml file in downloaded MOD folder 00_ART_MODS_MassImport.xml ( loads replacement art files)
checkbox ON "DirectSave" , click "Start" ( may take 1 minute )
the OUTPUT folder now contains the modded .mul files
UOFiddler Tabs >UOP Packer > paste the OUTPUT folder path that contains the modded .mul files > turn ON "Pack MUL to UOP" > Start
close UOFiddler , then copy the .mul , .idx , and .uop files , in the OUTPUT folder into your Ultima folder , overwriting
Install is complete , play Ultima Online
if your shard updates , then patch using their updater ( verify ), and repeat the MassImport process using UOFiddler after
Patching Troubleshooting
if Terrain textures are incorrect in-game showing mismatch random texture = this is typically a shard patch update applied on top of the modded , mod textures should be applied by MassImport AFTER updating to your shards latest
if MassImport crashes/freezes ( longer then 2min ) = the source .mul may be corrupted , restore backup .mul files or remove them and verify your shards latest patch ( so it downloads unmodded versions ) . this may happen after patching latest in a shard updater
if after a Shard Update the client crashes with "System.OutOfMemoryException: Array dimensions exceeded supported range." referencing the ArtLoader. this is likely a failed diff update , delete the .mul or .uop files in the client directly that were recently updated , then verify again with your shard's launcher to download a fresh copy. test that the client loads , then may MassImport ontop using UOFiddler