1) Move the "Minecraft-Omnia+" folder to C:\Games   (or wherever you want it, but update the JVM Arguments accordingly)
2) Run the forge install jar
3) Use the Minecraft launcher to make a new "installation" using the following settings

Version:   release 1.15.2-forge-31.2.45
Game Directory:  C:\Games\Minecraft-Omnia+
Java Executable:  <Use bundled Java runtime>
JVM Arguments:  -Xmx7G -Xms5G -XX:PermSize=4G -Dminecraft.applet.TargetDirectory="C:\Games\Minecraft-Omnia+" -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=64M

For custom game directory, be sure to change the direcotry in BOTH "Game Directory" and the "JVM Arguments" string.