First you need to have a premium account and some spare tier
If you've decided you'd like to donate please contact Wellington Beam for a group invite.
Once in the group, open up the 'My Groups' window using 'ctrl-g' or right click on yourself and select 'My Groups'
Right click on NCI Land Holdings and select 'View Info'
Click on the Land/Assets tab
Enter the amount of tier you'd like to donate in the text box. There should be a flashing cursor in it. The maximum amount you could donate without increasing your current tier level is indicated below the white box.