Open the script with any text editor (however, Notepad++ is recomended)

The config has only 3 tables:
The first table (startswith) filters any lines that start with defined symbol/word/sentence etc.

The second table (endswith) works like the first table, but catches any matches at the end of the line.

The last, third table (finds), uses the string.find function. You can define to look for normal strings (symbols,words) and you can also take advantage of the powerful regular expressions.

Blue lines mean that the text wasn't filtered and blocked by LogClener. They're still in logs.
Red lines show you which text was filtered and where it would be normally, without LogCleaner.

On the right you'll find the config I used for tests. The underlined the tables show you which text was filtered by that table. That also shows you where the script found a match.

A more interesting example is the first filter in 'finds':
it looks for an UPPER-CASE letter in the text. That's a regular expression. The pattern "%u" looks for an upper-case letter as I said above. There're some tutorials about regular expressions:
here: http://www.lua.org/pil/20.2.html
here: http://www.wowwiki.com/Pattern_matching
and here: http://lua-users.org/wiki/PatternsTutorial
(Attention: Lua uses a different regular expression engine than most other languages do!)

Matches:
rofl I hate cookies
fuck you!

But not:
r0fl I hate cookies
uck you!
you fuck

Matches:
come here, Darklord
it's okay to be gay

But not:
you're gay!
lord stop it

Matches: (any upper-case letters, 1337)
... Fuck you this is caps
haha CAPS lol
what is cool? 1337 is cool!
1337 ftw!

But not:
hello!
0245689
abcdef etc.

I hope it's enough. Ask in comments for help!