For Kirikiri there is a guide here.
As for other engines, you basically just have to extract the script, translate it and put it back in.
For some engines there are tools out there, for the other you will have to do it yourself.
You should be able to extract the English Scripts just like the Japanese ones from the patched games.
However, you can also extract the Japanese script and and play both versions at the same time while translating.
It sounds retarded(which it probably is), but not translating from the source is already pretty stupid anyways.