Install the extension from the Typo3 Extension Repository (TER)

You can change the appearance of the search, by simply creating a new template-file. There is a default template-file included in the extension (in the folder EXT:ch_is_templates/search.html) which is a copy of the standard search design. If you want to modify the template, take a look at that file, make a copy of it and modify it to your own design. Observe the comments in the file - there are markers that define where content is inserted and which parts are used for this and that. Self explanatory to a certain degree...

In your TS template place this code and change the default value:

plugin.tx_indexedsearch.templateFile = EXT:ch_is_templates/search.html