How and when to use patch.
You know it's time to ditch sed and use patch when you have say 4 or more lines of sed or it is very hard to read. I would still use sed in some cases such as using the system CFLAGS/CXXFLAGS in a static file on those ports that lack a configure script. Note that some packages can use example: ./build.sh CFLAGS="$CFLAGS".
Here is how I use patch and diff.
Once you have your sources extracted cd into the directory and clone it.
$ cp -r foo-1.2.3 foo-1.2.3.orig
Edit the files in foo-1.2.3 only keeping foo-1.2.3.orig as is then make sure you remove any backup files if you got any *~. Now to make a patch type this command.
$ diff -pruN foo-1.2.3.orig foo-1.2.3
That will print it to the console, once you are happy pipe it to a file name.
$ diff -pruN foo-1.2.3.orig foo-1.2.3 > foo-1.2.3-some-change.patch
To place it in a upper level directory do this. Keep adding ../ as needed or use a full path.
$ diff -pruN foo-1.2.3.orig foo-1.2.3 > ../foo-1.2.3-some-change.patch
Now in your Pkgfile you can do this.
patch -p 1 -i $SRC/foo-$version-some-change.patch
Be sure to add foo-$version-some-change.patch to the source array.
After all that you can remove or reverse the patch you just done on foo-1.2.3 and repeat as needed making additional patches. It is best to separate patches for easy maintenance, by grouping similar changes in the one patch.