I need help with making a custom item aim like a gun

[nikolaq]

I am using Mcreator to make a model of a gun and i managed to import it intro Minecraft since its easy, now what i need to do is make it so i hold a gun like when charging a bow like shown in this video https://gyazo.com/de87cfcb79dea7b565013077cb77bb04, i did a lot of searching and found out its impossible to do with Mcreator and i need to code it in, after some more searching and looking again and again trough this forum post 

 

It has shown a lot of code, most of it i cant understand and then in the end person said event.getRenderer().getEntityModel().rightArmPose=ArmPose.BOW_AND_ARROW; is the thing that is going to work, so now im tryng to find out how to make a player start this even when holding a custom item, is there any help or tips you can give which will help me.