download whatever revision from SVN
download patchfile and put in the root directory of the downloaded svn tree
patch -p0 < patchfile.patch
If it fails saying "can't find file, here's the first 5 lines blah blah", change -p0 to p1. If same, try p2
If one or more "hunks" fails, then you're trying to apply it to a revision that it wasn't made for and something about the code it's looking for has changed and thus the patch isn't compatable.
Applying 2 patches at once is fine, with the caveat that if they are trying to modify a same bit of code, the second will have the "hunks" failing error and it won't work.
.patchfiles are human-readable. If something fails and you are ambitious, you can often go "apply" it manually yourself to the code.