Wireshark dissector: There is a header before Ethernet packets

I have my own protocol but I don't know how to create my own wireshark dissector.. Ethernet packets looks like:

[Dst MacAddress][Src MacAddress][Ethertype].....

My protocol add a header:

[My protocol header][Dst MacAddress][Src MacAddress][Ethertype].....

I want to dissect the header and use the Ethernet Protocol to dissect the remaining, what should I do?