1. Log into your Blogger.com account.
2. Click the Layout tab
3. Click Page Elements.
4. Click Add a Gadget
5. Click the plus sign next to HTML/Javascript.
6. Type Search this site in the Title box
7. Copy and paste this code in the text area.

<p align="left">
<form id="searchthis" action="BLOG-URL/search" style="display:inline;" method="get">
<strong>SEARCH HERE

<br/></strong>
<input id="b-query" maxlength="255" name="q" size="20" type="text"/>
<input id="b-searchbtn" value="Search" type="submit"/>
</form></p>

8. Click Save
9. Click Preview