Inserting new code caves inside sections of windows PE malware files

Question

I am trying to insert the new code caves inside windows malware PE files by expanding the size of existing sections. I was able to dig out the existing code caves in the file but expanding the size of existing sections to insert code caves (just the empty space) is confusing me a lot as it can easily break the file. Can someone help me with the details I should be looking at to insert those empty spaces which are not initially present inside the file?