Simple. At least in Debian, Kubuntu and other Debian-derivatives. Install Samba server and the appropriate KDE4 module as root:

# apt-get install samba kdenetwork-filesharing

The kdenetwork-filesharing package installs a nice System Settings panel to configure network file-sharing using NFS and Samba. It also adds (the initially missing) functionality in the Properties > Share tab of each file and folder in Dolphin.

So, install the above packages, then open Dolphin, select a folder, right-click on it and select Properties then go to Share tab. Click on Configure File Sharing button and a new dialog appears. Let the “Simple Sharing” option intact and click on the Add button. A new dialog appears where you select and add the desired folder. Click OK twice. You are done.