You don't. LSI is a mathematical algorithm for establishing the relationship of any word to any and every other word. No search engine uses it - none of them can make such an encompassing algorithm workable - Google uses much cruder forms of semantic matching and has taken out several patents on it - the main use has been in adwords. The way that webmasters can/should react to semantic matching is by using words that are synonyms to their main keyword, and other words that one would naturally expect to find in the same context, in your content and other places you use your keywords. In other words, write good natural content, for readers, using a wide vocabulary, but not in a manner that makes it look like you've swallowed a thesaurus. Just write in good English, really - or French or Spanish or Hindi or whatever language you write in.