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.

jQuery Popup Boxes to Blogs/WebSites

<script src='https://ajax.googleapis.com/ajax/libs/jquery/1.6.4/jquery.min.js' type='text/javascript'></script>
<script type='text/javascript'>
//<![CDATA[

/***************************/
//@Author: Adrian "yEnS" Mato Gondelle
//@website: www.yensdesign.com
//@email: yensamg@gmail.com
//@license: Feel free to use it, but keep this credits please!					
/***************************/

//SETTING UP OUR POPUP
//0 means disabled; 1 means enabled;
var popupStatus = 0;

//loading popup with jQuery magic!
function loadPopup(){
	//loads popup only if it is disabled
	if(popupStatus==0){
		$("#backgroundPopup").css({
			"opacity": "0.7"
		});
		$("#backgroundPopup").fadeIn("slow");
		$("#popupContact").fadeIn("slow");
		popupStatus = 1;
	}
}

//disabling popup with jQuery magic!
function disablePopup(){
	//disables popup only if it is enabled
	if(popupStatus==1){
		$("#backgroundPopup").fadeOut("slow");
		$("#popupContact").fadeOut("slow");
		popupStatus = 0;
	}
}

//centering popup
function centerPopup(){
	//request data for centering
	var windowWidth = document.documentElement.clientWidth;
	var windowHeight = document.documentElement.clientHeight;
	var popupHeight = $("#popupContact").height();
	var popupWidth = $("#popupContact").width();
	//centering
	$("#popupContact").css({
		"position": "absolute",
		"top": windowHeight/2-popupHeight/2,
		"left": windowWidth/2-popupWidth/2
	});
	//only need force for IE6

	$("#backgroundPopup").css({
		"height": windowHeight
	});

}

//CONTROLLING EVENTS IN jQuery
$(document).ready(function(){

	//LOADING POPUP
	//Click the button event!
	$("#button").click(function(){
		//centering with css
		centerPopup();
		//load popup
		loadPopup();
	});

	//CLOSING POPUP
	//Click the x event!
	$("#popupContactClose").click(function(){
		disablePopup();
	});
	//Click out event!
	$("#backgroundPopup").click(function(){
		disablePopup();
	});
	//Press Escape event!
	$(document).keypress(function(e){
		if(e.keyCode==27 && popupStatus==1){
			disablePopup();
		}
	});

});

//]]>
</script>
<style type='text/css'>
#backgroundPopup{
display:none;
position:fixed;
_position:absolute; /* hack for internet explorer 6*/
height:100%;
width:100%;
top:0;
left:0;
background:#000000;
border:1px solid #cecece;
z-index:1;
}
#popupContact{
display:none;
position:fixed;
_position:absolute; /* hack for internet explorer 6*/
height:450px;
width:500px;
background:#FFFFFF;
border:2px solid #cecece;
z-index:2;
padding:12px;
font-size:13px;
}
#popupContact h1{
text-align:left;
color:#6FA5FD;
font-size:22px;
font-weight:700;
border-bottom:1px dotted #D3D3D3;
padding-bottom:2px;
margin-bottom:20px;
}
#popupContactClose{
font-size:14px;
line-height:14px;
right:6px;
top:4px;
position:absolute;
color:#6fa5fd;
font-weight:700;
display:block;
}
#button{
text-align:center;
margin:100px;
}
</style>

Note:You can change width and height of popup box,colors,.... as your choice.

4.Now save your template.

5.Go to Layout-->Page Elements and click on "Add a gadget".

6.Select "html/java script" and add the code given below and click save.

<div id="button"><input type="submit" value="Your-Popup-Button-Name" /></div>

<div id="popupContact">

<a id="popupContactClose">x</a>

<h1>Enter-Popup-Title-Here</h1>

<p id="contactArea">

Enter-Your-Popup-Content-Here

</p>
</div>

<div id="backgroundPopup"></div>

Note:Replace Your-Popup-Button-Name,Enter-Popup-Title-Here,Enter-Your-Popup-Content-Here with your data.

You are done.

Demo