I want to make a "codecave" in an executable file using OllyDbg, but my issue is that I have no idea how can I add more space where I could write my own function to the file. So far I've only tried to extend it by adding zeros to the end of the file using a Hex editor, but after reopening it with Olly I didn't got more space for instructions.
Strangely I couldn't find any information about this topic on the internet, so I hope some of you here can help me out.