Upload the folder sendcard/ and all subfolders/files to your website.

If you are on a Unix/Linux/FreeBSD server chmod the files sendcard.php, 
sendcard_setup.php, admin/config.php, admin/plugins/<all files> and
include/lang/<all files> to 666 (that is, rw-rw-rw) so that the web server 
can write to them.  To do this, you must either use your FTP program
and do each file manually, or if you have ssh access to your server
you can use the script chmod.php to do it for you.  Run it via the
command line:
% php -q ./chmod.php


Go to http://yourdomain/sendcard/admin/index.php and log in using the
password:
  password

Click on the link for the installation wizard and follow through the
steps.  Don't forget to change your password!

If you get stuck, don't forget the support forum at 
http://www.sendcard.org/forum/.  Please search the forum before posting
your question - I get fed up answering the same questions again and 
again.

Full documentation on customising sendcard can be found at 
http://www.sendcard.org/documentation/