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Guestbooks rely on script files in order to collect and display responses. Since you don't have the ability to upload scripts to Blogger blogs, you'll need to link to a guestbook that is hosted on another site. This will require you to create an account on this site so that you can manage and store your guestbook. There are free and paid options, depending on your needs. You'll be generating your guestbook on another website, and then copying the code for the book into a new page on your blog. There are a variety of different services available that can generate and host a guestbook. Different services will offer different features. A few of the most popular services are listed below, and most are free:  Bravenet - Bravenet.com offers free guestbooks, and you can choose different designs and add smilies for guests to use. The Bravenet guestbook will appear directly on your blog. Bravenet gives you the most control over your guestbook design out of all of the options listed here. 123Guestbook - This free guestbook only takes a few moments to setup,but offers less customization than Bravenet. Ultraguest - This guestbook has a lot of moderation features, including private messaging. SnapHost - SnapHost is a paid guestbook service that offers CAPTCHA and anti-spam functions. It's actually a CAPTCHA service first and foremost that includes a guestbook among its features. Choose this option if the other features appeal to you as well, otherwise it's probably not worth it for just the guestbook. All of the services listed above require you to create an account. You'll use this account to manage your guestbook settings, since they can't be managed from your blog. This is because the actual guestbook database is located on the guestbook service's servers. You'll be prompted to enter your blog's address when creating your account. This will allow the service to connect to assign the guestbook to your website so that replies are properly tracked. Once you have created your account, you can begin customizing your guestbook. Depending on the service, you may have to open the guestbook's control panel. For example, 123Guestbook will send you the link to the control panel in your registration confirmation email. If you're using Bravenet, you'll need to click the "More Web Tools" link on your dashboard, and then click "Register" next to "Guestbook". Use the guestbook editor or control panel to customize it to your liking. If you're using Bravenet, you can edit all of the colors so that it matches the theme of your Blogger blog exactly. You can typically choose from a variety of preset themes that will make setting up the guestbook quicker. Get the code for your guestbook. After you've finished customizing your guestbook, you'll want to grab the embed code for it so that you can add it to your blog. In Bravenet, click the "Get Code" link to open a new window. Select "Get Code to Embed" and then copy the code from the box. If you selected 123Guestbook, you'll need to create a link to the guestbook's URL. You can find the code for the link in the "Extras" section control panel menu. Putting your guestbook on a page instead of a post will allow you to easily link to it or create a menu option for it. Log into Blogger, select your blog, and click the "Pages" option. Click "New page" to open a new blank page. This code will embed the guestbook into your page. You won't be able to see it in Preview mode. Turn off Reader Comments in the page's Settings menu to remove the comment box from the page so that it doesn't clash with the guestbook. Once you've published the page, you'll be able to visit it and see your guestbook. Any visitor that comes to this page will see the guestbook. You can link to this page or add it to your blog's navigation bar so that visitors can easily find it. To quickly add a navigation menu, select the "Layout" option and then click "Add gadget". Select "Pages" from the list of gadgets to add a navigation bar.

Summary:
Understand the process. Decide on a guestbook service. Create an account. Create your guestbook. Customize your guestbook. Get the link to the guestbook. Create a new page on your blog. Switch to HTML mode and paste the code. Publish the page.