Once the TextMate is installed. First, I install the GetBundles bundle using the commands below from my Terminal.

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mkdir -p ~/Library/Application\ Support/TextMate/Bundles
cd !$
svn co http://svn.textmate.org/trunk/Review/Bundles/GetBundles.tmbundle/
osascript -e 'tell app "TextMate" to reload bundles'

Then I goto Bundles > GetBundles > Get Bundles and update my outdated bundles by pressing ⌘+U (Install all Updates). After that, I install the following bundles -

LESS - a textmate bundle for less css using the node version of the library at http://github.com/cloudhead/less.js (by Tim Caswell)