Board Thread:LotR Mod Technical Support/@comment-75.136.40.112-20170201030940/@comment-25330335-20170202002700

Flags are the command-line arguments that the launcher passes to Minecraft.jar when it runs. If you edit your profile in the vanilla launcher, then there should be a section at the bottom of the dialogue window that has JVM flags. You just need to copy these from that text box to the JVM flags box in MultiMC, which can be found in instance settings.

I hope that makes sense...