Render New Symbols at Task and MessageFlow

I used this example:
for extending my meta model.
Now I want to draw Symbols for my extension. Some at tasks, but they should be there together with symbols for Usertask etc.
Ans some should be at Message Flows.
I already looked at some Render examples, but I think I need a Step-by-step introduction.

Can you describe your use case more in detail? It will hard to give you a “step-by-step” introduction without knowing your use case.