First, if you havn't downloaded ImpressCMS yet, you should do it right now. You can always find the latest version on http://www.impresscms.org
After you have extracted an archive, and entered the 'xxx' folder, you can see 4 directories:
docs - holds this document as well as license, changelog and language_changelog which is very useful for translators
extras - holds a login file if you wish to use ImpressCMS on a ssl
based server. (If other files are included here, they will have their
own details)
htdocs - contains fundamental files of our software
upgrade - it's content will make happy everyone who would like to
switch from XOOPS to ImpressCMS or upgrade from older ImpressCMS version
Once you have known htdocs as our working place, you need to upload
it to server. Using your FTP client move the contents of htdocs
directory to desired place on server (often it is server root
directory). The rest of the downloaded package is not necessary to
install ImpressCMS so they won't be useful for us any longer.
Before we start the installation process, there are several things
you need to know.
1. The essential issue is the server environment. In order to get
ImpressCMS working properly, your server has to meet these
requirements:
HTTP Server (Apache, IIS, Roxen, etc.)
PHP 4.3.0 or higher (For ICMS 1.0.x branch) (5.1 or higher recommended)
As of ICMS 1.1.x We no longer support PHP 4. So PHP 5.2.5 or higher is required. More info
MySQL 3.23 (4.1 or higher recommended)
2. Since you want to have content, you need a place to keep it. Please prepare the following information:
database server
database name
database user
database passwordSometimes happens that [it's not correct in
English I suppose] database user doesn't have proper priviledges. This
may cause problems, so watch out for this.
3. After that, please make sure your browser accepts cookies and you
have JavaScript enabled.
Show/Hide help
During the installer, you will be able to display useful help and information about the process, by clicking the button below.

Advanced
users may not require this information, but we strongly recommend its
usage for people new to ImpressCMS, as it reveals solutions for many
problems you may otherwise have.
Installation
Once the files are copied there, you can start the install by typing http://yoursite.com. (http://localhost if you run it on local server). We are automatically redirected to installation script, which will begin the install process.
Step 1: Language

In the first step of installation we can choose a language. There are
as many possibilities as many language folders are under the
/install/language/ directory. Select language and click the Next button.
Step 2: Welcome screen

In this step we can read a very basic information about ImpressCMS and
its license. There are also some text which I have already explained
and some which I will explain further. Next.
Step 3: Checking server configuration

In the third step the script will check the server configuration.
This will show the details of your server, and if they are suitable for
your installation. If the text is green, then everything is good.
Step 4: Paths and permissions

The next part of the install Wizard is designed to check your file and
directory permissions. If your running in a Windows environment, this
should be a pretty painless step. I use Linux here and as the above
screenshot shows, there are problems with permissions. In order to meet
required permission you have to change them as Wizard is telling you in
a tip box. Give CHMOD 777 to mainfile.php, /templates_c, /cache,
/uploads And hit Refresh button.
Phisical path and Web locations address will be defined by script
itself. If you want to learn a bit more about them, click Show/hide
help button.

If
you did last action properly, you should get a message like on the
screen above. Now, the only thing you have to set up is Trust path. To
get more info about this click the Show/hide help button. Click help
button for more informations This folder should be writable (CHMOD
777). [Perhaps more info about trustpath needed here I suppose]

After we set up the Trust path and script said "Trust path found", we can move forward to the next step.
Step 5: Database configuration

Let's
begin with a database. Server hostname - if your hosting provider
hasn't told you different - is localhost. Simply type it there.
It is time to type the rest of info which I told you to prepare in the
beginning. Table prefix is generated randomly to ensure good protection
against hackers. After all we click Next.
Step 6: Saving configuration

In this step thing which you have given before are saved into file. mainfle? secret file under trust path?
Step 7:Tables creation

First part of this step is telling us that it hasn't found any
ImpressCMS tables and it is ready to create clean tables which will be
structure for whole databas

In the second part we can see which tables were created. These tables are empty now but we will fill them soon.
Step 8: Admin account

Please
be careful here and write down or remember your administrative
password. You will need this after the install to continue setting up
your site. Once you have completed inputting the correct information,
click Next to continue.
Step 9: Inserting initial data
As we can see script is ready to insert first data into our database. There is nothing to do here. Next.

In the second part of this step Wizard is inserting default ImpressCMS
values in to previously created tables. These values let ImpressCMS the
essential working.
Step 10: Final

This is last step of installation. You can go directly to your brand
new website by pressing Show my site button.
If all went well, then your new site should be up and running. If not,
please visit community.impresscms.org for further assistance.
Anyway, I suggest you looking at links given. It is worth checking them
every time you have any problem, need support, or want to help others.
See you there :)