First of all the question "why do this online when I can send a letter?" - The reason for setting up a blog is to get noticed, I was ignored for 6 weeks before I setup my blog, and then I contact the company by Twitter telling them about my success in Google over their company and soon enough I was being contacted by a member of the customer services management discussing compensation over the phone. This I feel is down to a few factors but the most prominent being that the internet is now incredibly important for companies big or small, and bad publicity online can be devastating as it allows thousands or potentially millions of people to view it everyday, and especially in the current financial climate anything like this is too big to be ignored and as such gets very immediate attention as soon as the companies media or internet comms department see it ( You can help them find it by posting it all over their twitter page or Facebook group ).
Back to writing your own free blog and how to setup everything you need fully with SEO, now I do apologise if I have explained in too much of a technical manner (I am known for doing this) if I have just point it out in the comments and I'll explain it better as I understand everyone isn't technically able.
You will need to create a free wordpress blog account. The first part of registration asks for basic details such as username, password, and email (please read the terms and conditions) the next page will asked for "blog domain" I would suggest using something that is relevant to your content. Such as my-portfolio.wordpress.com or sponsor-me-online.wordpress.com additionally with the 'blog title' keep it simple and related to something you wish to be found under on Google. the 'Privacy' box you need ticked else Google will not look at your site.
After this you will receive an e-mail to confirm the new account you have created, there will be a link provided in the email and it will take you through to confirmation, and then through to a page that looks like the image below, where you will need to type in the username and password you specified earlier during registration.
Once you have entered in the correct details into this area you will be forwarded to the screen below, don't worry in order to get started you only really need to use the "posts" area, ofcourse you can start to play with categories, tags, templates in the future. But for now you only need to use the posts, to get content flowing. Posts is on the left hand side directly underneath where 'Upgrades' is, click on this to get to the area where you add/edit and delete content.
When you click on 'Posts' you will be presented by the following screen, as you can see there is already a sample post there, ignore this for now as you can come back and delete it at a later date. What you need to click on is an item on the left hand menu which has appeared since you clicked on 'Posts'. You now need to click on 'Add New' which is just below Posts.
After you have clicked on 'Add New' you will be presented with the following screen, except yours will have no content, enter in a title (make sure its meaningfull - but short) and some content below make sure this talks about the key values of what your website is about, and that it mentions what items you are trying to get higher up on Google with on your blog.
On the right hand side, you can see a blue button which has "Publish" on it, when you have finished writing your first post (remember you can, and should write many more) press this button to make the world able to see your post. After you have finished this you will need to open a new window or tab on your browser and type in the blogname you specified earlier so if you called it udjamaflip it would be http://udjamaflip.wordpress.com once you have navigated to this page (which is the front end of your new blog) you can see your shiny new blog post and it should look similiar to the screenshot below.
Just to further my previuos points, a blog should not be your sole method of complaining against a company however it is a powerful last resort. You will be required to continue keeping content rolling in, and make sure it is meaningful and continues to use important phrases you plan to come high on Google with, this doesn't mean spam "complain against xxxxxx" through your content, it just means structure your content methodically, make the content so it is made for a human being to read.
Additionally I must mention that SEO is something that doesn't come overnight, and as such, requires a lot of time and effort and learning how to use Google Analytics, Google webmaster tools, and more advanced SEO involvement within your website, such as meta tags, interlinking and much more. If you wish to have a blog with this kind of professionalism and care it is going to be necessary for your to invest in a web design company and SEO specialists who can use their knowledge to help your website.