How can i edit the style of my own map? with tilemill and Mapnik???

Hi there.

I have tried with this site'http://switch2osm.org/serving-tiles/building-a-tile-server-from-packages/'

And i can run the tile server.

After that i want to change the style of map. so i tried with Tilemill for editing with new installed ubunto. i just export one of the example(The D.C) to MAPNIK XML. also copied that file in to 'etc/mapnik-osm-carto-data'

and i do run this'osm2pgsql --slim -C 1500 --number-processes 4 ireland-and-northern-ireland.osm.pbf'

but i got just pink tiles for this.

Is that wrong process for editing my map??????

if i'm wrong, what should I do to edit style with Tilemill and Mapnik???