The basic concept it to apply the 'diff' using the patch command.
Basic command syntax
$ patch -p[num] < [patchfile]
Whether or not this will work for your diff file will depend on what your patching and how the diff file is written. Recommend doing 'man patch' and 'man diff' and read the manual. Would also help you to look at any README files or HOW-To's about the specific use of the diff you trying to apply.
Hope this helps.