How can I customize different plot styles with "forest" function? Question from a non-programmer

[Monster-White]

1. How Can I customize my forest plot in gemtc?
   Such as replace a square with diamond to present the OR or RR, change the color of line, etc.
2. I was trying to patch two forest plots together by "cowplot" package. But after value assignment with following code, "p1" and "p2" presented as NULL, which limits my next step in patching process.
   p1 <- forest(relative.effect(results1,"2"),use.description=T)
   p2 <- forest(relative.effect(results2,"4"),use.description=T)