Manually Applying patches (HowTo)

NOTE: This post came about as a result of questions in a different thread, but it makes more sense to give it it's own thread with an appropriate title. It is based on part on Ben Garney's resource How to make Patches (Cygwin/Unix).

Patch files are broken down into sections, which is loosely defined as:
File header
1 or more "hunks"
(possibly more file headers with their own hunks)

Here is a structured example:

File description header ( 2 lines): the source of the patch is indicated by "---" and the target indicated by "+++".
(Note: All that really matters to you is the file name itself, and where it is in your dir structure).
Example:

--- RTSKit/engine/terrain/terrData.cc	2004-11-26 10:45:59.120822000 -0500
+++ rts-engine/engine/terrain/terrData.cc	2004-11-26 11:16:08.821706632 -0500

a "locater" line, indicated by "@@", which is uses to find out roughly where the code to be changed by this hunk is:

@@ -54,20 +54,24 @@

NOTE: This locator is used by the automated patch application to find the general location of the code that is to be changed. While you can use it to get started, it can be very difficult to use in a long patch that is applied manually, since they are running line numbers that can change as each patch is applied. Most people that apply patches manually do not use this, or only use it very loosely.
For general knowledge, what you have is two points of information, one for the original file, and one for the target file. The first number in each point (the 54) indicates a location line number, and the second number (20 for the "-" datapoint, 24 for the "+" datapoint) is the +/- offset that the code can be found in. In other words, to find the code for this hunk:
in the source file ("-"): go to line 54, and look within +/- 20 lines (could be 10, or half the number, I honestly don't remember) to find it.
in the target file ("+"): go to line 54, and look within +/- 24 lines.

"Reference" code: These are actual lines of code from the file being changed, but do not have a "+" or "-" in front of them. They are used by the automated process to lock down the exact place to change the code, and should be used the same way in a manual patch application. Most of the time, you can use "search" on the entire line to find the right place to modify code, just sanity check it before blindly applying changes.

//--------------------------------------
 void TerrainBlock::setFile(Resource<TerrainFile> terr)
 {

"Change" lines: These are the actual changes to your code. They operate on an "entire line" basis, and have two types:
- : delete whatever line in your file is shown after the -
+ : add whatever line is shown in the patch file to your file.

-   mChunkMap = mFile->mChunkMap;
-   materialMap = mFile->mMaterialMap;
-   heightMap   = mFile->mHeightMap;
-   flagMap = mFile->mFlagMap;
+   mChunkMap        = mFile->mChunkMap;
+   materialMap      = mFile->mMaterialMap;
+   heightMap        = mFile->mHeightMap;
+   flagMap          = mFile->mFlagMap;

NOTE: In this particular example, if you analyze what's really changed, all you will see is that we add some whitespace between the variable name and the = sign. Due to the nature of automated patching, that requires you to delete the entire line, and then copy in the new line. A bit overkill in this particular case, but it's for automated processing of the patch.
(cont)