1.Login to your blogger dashboard--> layout- -> Edit HTML

2.Scroll down to where you see </head> tag .

3.Copy below code and paste it just before the </head> tag and save your template.

<style type='text/css'>
a.tt{
position:relative;
z-index:24;
color:#3CA3FF;
font-weight:bold;
text-decoration:none;
}
a.tt span{ display: none; }

/*background:; ie hack, something must be changed in a for ie to execute it*/
a.tt:hover{ z-index:25; color: #aaaaff; background:;}
a.tt:hover span.tooltip{
display:block;
position:absolute;
top:0px; left:0;
padding: 15px 0 0 0;
width:200px;
color: #993300;
text-align: center;
filter: alpha(opacity:90);
KHTMLOpacity: 0.90;
MozOpacity: 0.90;
opacity: 0.90;
}
a.tt:hover span.top{
display: block;
padding: 30px 8px 0;
background: url(http://4.bp.blogspot.com/_4HKUHirY_2U/Sv_TE02nC2I/AAAAAAAAALU/RJP8dN5W30s/bubble.gif) no-repeat top;
}
a.tt:hover span.middle{ /* different middle bg for stretch */
display: block;
padding: 0 8px;
background: url(http://3.bp.blogspot.com/_4HKUHirY_2U/Sv_TISo06uI/AAAAAAAAALc/9dj1-t3hN6o/bubble_filler.gif) repeat bottom;
}
a.tt:hover span.bottom{
display: block;
padding:3px 8px 10px;
color: #548912;
background: url(http://4.bp.blogspot.com/_4HKUHirY_2U/Sv_TE02nC2I/AAAAAAAAALU/RJP8dN5W30s/bubble.gif) no-repeat bottom;
}
</style>

4.Now when you add a link use the below structure:

<a href="YOUR-LINK" class="tt">LINK-NAME<span class="tooltip"><span class="top"></span><span class="middle">YOUR-TOOLTIP-MESSAGE</span><span class="bottom"></span></span></a>

Look at the example below.

<a href="http://www.premiumbloggertemplates.com/" class="tt">Premium Blogger Templates<span class="tooltip"><span class="top"></span><span class="middle">Download Free Premium Templates Free</span><span class="bottom"></span></span></a>

It will look like this:

CSS Bubble Tooltips

source