After cloning the void-packages repository the first thing you need to do 
is create the bootstrap environment:

./xbps-src binary-bootstrap

The next step depends on what you want to achieve. Do you want to update 
the package for future use by you and/or others? Then the cleanest way 
would be to put a patch-file into "srcpkgs/<packagename>/patches/". Then 
increase the revision in "srcpkgs/<packagename>/template". This is of 
course the tricky part. Afterwards you can build an dinstall the package as 
you have written. Just add the package name at the end of your xbps-install 
line :)

If you only want to make the changes this one time and not preserve them 
you can get xbps-src to get the sources for you:

./xbps-src extract <packagename>

Edit the the sources as desired in 
"masterdir/builddir/<packagename+version>/"

Build and install like this:

./xbps-src -C -f pkg <packagename>
xbps-install -f --repository=/path/to/hostdir/binpkgs <packagename>