Magento Improved horizontal checkout css steps

If you want to change Magento  default vertically checkout step to Magento horizontal checkout step
. than don't worry simple follow below step.

Step1:- open you're styles.css file
Path:- skin\frontend\default\<theme name>\css\styles.css

Step2:- At the end of file paste the below code

/*********  Start Magento Improved horizontal One Page Checkout css **********/

.block-progress .block-title { background:none; margin:0 0 34px; padding:0; }
.checkout-onepage-index .col-main { padding:0; border:0; }
.checkout-onepage-index .page-title { padding-right:40px; width:auto; }
.checkout-onepage-index .page-title h1 { background:none; padding:0; }

.opc { position:relative; overflow:hidden; height:970px; padding-top:20px; text-align:center; border:1px solid #BBAFA0; background:#F9F3E3; }
.opc .buttons-set { margin:15px 0 0; opacity:1!important; }
.opc .buttons-set p.required { margin:0; padding:0 0 10px; }
.opc .buttons-set .back-link { display:none; }
.opc .buttons-set .please-wait { position:absolute; z-index:99; top:30%; left:50%; margin:-80px 0 0 -146px; border:5px solid #f3b66f; font-size:12px; background:#fff; padding:30px; white-space:nowrap; border:1px solid #c0c0c0; -moz-border-radius:6px; -webkit-border-radius:6px; border-radius:6px; -moz-box-shadow:1px 1px 2px rgba(0,0,0,0.2); -webkit-box-shadow:0 0 50px rgba(0,0,0,0.2); box-shadow:0 0 50px rgba(0,0,0,0.2); }
.opc .buttons-set a { color:#214D90; }
.opc .ul { list-style:disc outside; padding-left:18px; }
.opc li.section { display:inline; }
.opc .step-title,.opc .allow .step-title { border:1px solid #F9F3E3; height:69px }
.opc .step { padding:30px 20px; position:absolute; border:0; top:110px; left:0; z-index:1; background:#FBFAF6; width:605px; height:900px; border-bottom:1px dotted #ccc; border:none; width:643px; text-align:left; border-top:1px solid #BBAFA0; }
.opc .step-title .number,.opc .allow .step-title .number,.opc .active .step-title .number { float:none; display:block; margin:0 auto; margin-bottom:10px; }
.opc .step-title { position:relative; float:left; text-align:center; padding:15px 11px 0; margin-left:-1px; background:none; }
.opc .step-title a { display:none; }
.opc .step-title .number { background:#ccc; color:#fff; width:30px; height:30px; line-height:30px; -moz-border-radius:20px; -webkit-border-radius:20px; border-radius:20px; margin-bottom:10px; padding:0; border:0; }
.opc .step-title h2 { font-size:12px; color:#bbb; clear:both; }
.opc .allow .step-title { cursor:pointer; background:none; }
.opc .allow .step-title .number { background:#000; color:#fff; }
.opc .allow .step-title h2 { color:#000; }
.opc .active .step-title { color:#ef0606; cursor:default; border:1px solid #BBAFA0; border-bottom:1px solid #FBFAF6; z-index:2; background:#FBFAF6 url(../images/bkg_checkout.gif) 0 0 repeat-x; }
.opc .active .step-title .number { background:#F18200; border-color:#fff; color:#fff; }
.opc .active .step-title h2 { color:#F18200; }
.opc .step-title h2,.opc .allow .step-title h2,.opc .active .step-title h2 { width:100%; text-align:center; }
.opc .step-title { width:16%; }
.opc .form-list .field,.opc .form-list .wide { }
.opc .form-list li fieldset { margin-bottom:40px; }
.opc .form-list label { float:left; width:220px; text-align:right; padding:4px 0 0; }
.opc .form-list label.required em { float:none; position:relative; right:4px; }
.opc .form-list li.fields { margin:0; }
.opc .form-list div.fields { width:100%; overflow:hidden; margin:0 0 8px; }
.opc .form-list .wide,.opc .form-list li.fields .field { width:645px; margin:0 0 8px; }
.opc .form-list li.wide .input-box,.opc .form-list .input-box { clear:none; float:right; margin-right:140px; width:260px; }
.opc .form-list li.wide select { width:390px; }
.opc .form-list li.wide input.input-text { width:254px; }
.opc .form-list .control input { margin:8px 0 0 25px; }
.opc .form-list li.control label { float:left; }
.opc form .form-list li.wide { margin-bottom:8px; }
.opc form .address-select { margin:8px 0 40px; }

.opc:first-of-type .step{-webkit-transition: all 0.5s ease-in-out;-moz-transition: all 0.5s ease-in-out;-o-transition: all 0.5s ease-in-out;-webkit-transition: all 0.5s ease-in-out;transition: all 0.5s ease-in-out;}
.opc:first-of-type .step[style*='display:none;'],
.opc:first-of-type .step[style*='display: none;']{display:block !important;}
.opc:first-of-type .section .step{left: 100%;}
.opc:first-of-type .allow .step{left: -100%;}
.opc:first-of-type .active .step{left:0;}
.opc:first-of-type li:last-child .step-title a{width:50%}
.opc:first-of-type li:first-child .step-title a{width:50%;left:50%}

#opc-login p.required { display:none; }
#opc-login h3 { margin-bottom:18px; }
#opc-login .col-2 { float:left; }
#opc-login .col-2 form fieldset h4 { display:none; }
#opc-login .col-2 { display:block; width:300px; }
#opc-login .col-2 fieldset { width:300px; }
#opc-login .col-2 .input-box { width:188px; margin:0; float:left; }
#opc-login .col-2 .input-text { width:182px; }
#opc-login .col-2 .form-list label { text-align:left; }
#opc-login .col-2 .form-list label { width:110px; }
#opc-login .form-list li.control label { float:none; }
#opc-login .form-list li.control input { margin:0 8px 0 0; }
#opc-login .col-1 { float:right; }
#opc-login .col-1 { width:290px; }
#opc-login .col-1 h4,#opc-login .col-1 .ul { display:none; }
#checkout-step-review.step { width:645px; }

#opc-review .step { padding:30px 20px; border-top:1px solid #BBAFA0; }
#opc-review .product-name { font-weight:bold; color:#0a263c; }
#opc-review .item-options { margin:5px 0 0; }
#opc-review .buttons-set { padding:20px 0; border:0; }
#opc-review .buttons-set p { margin:0; line-height:40px; }
#opc-review .buttons-set .please-wait { height:40px; line-height:40px; }
#opc-shipping_method .buttons-set { border-top:1px solid #E4E4E4; }

/*********  End Magento Improved horizontal One Page Checkout css **********/

Step3:- Save this file and delete cache folder inside var/ directory. Now see you're checkout step.