You can use the "Custom Post Template plugin" to select individual templates for your posts/portfolio entries. You can download it here:
http://wordpress.org/extend/plugins/custom-post-template/
Unzip the file you downloaded and open 'custom-post-templates.php'.
On line 48 change following code:
$this->add_meta_box( 'select_post_template', __( 'Post Template', 'custom-post-templates' ), 'select_post_template', 'post', 'side', 'default' );to:
$this->add_meta_box( 'select_post_template', __( 'Post Template', 'custom-post-templates' ), 'select_post_template', 'portfolio', 'side', 'default' );Afterwards upload the plugin files to your wordpress „plugins“ directory (located under wp-content/plugins).
Now you need to create a template:
a) Just make a copy of single.php, rename it to fullwidth_post.php and paste the following code before the first code line.
<?php
/*
Template Name Posts: Fullwidth
*/
?>b) Change following code in fullwidth_post.php:
<div class="wrapper" id='wrapper_main'>
<div class="center">to
<div class="wrapper fullwidth" id='wrapper_main'>
<div class="center">and delete following code:
<?php
$k_option['showSidebar'] = 'blog';
get_sidebar(); ?>
Create a new thread and tell me what problems you are encountering and what you want to know. 
There is no step 3 :) 
