You really should use the search button in these forums... :/

Anyway, the best way to install is to go into terminal (click the terminal application in your N900), type in "root" to be root (you need to install an application via application manager) and go to the folder where the .deb files are. Then, type:

dpkg -i <filename>.deb

And this should install the application. If it gives you errors about missing libraries, just type in (yes, also as root):

apt-get -f install

This should install all the missing dependencies.

Hope this helps and really, you should learn how to use the search button. Also, I am making the assumption you are using an N900 in this case.

Have fun!