Mainly, you need to add a COPYING file with the full text of the GPLv3 license, and add a license header to each source code file.
The header should contain this text: (replace year with the year[s] the project is/was active in, eg. 2011-2012, replace Full Name with your full name, and replace email with your email address)
Copyright (C) year Full name <email>
This file is part of NoLagg.
NoLagg is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
NoLagg is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with Foobar. If not, see <http://www.gnu.org/licenses/>.
Example copyright line as seen in one of my projects:
Copyright (C) 2010-2011 Elad Alfassa elad@fedoraproject.org
Your bukkitdev page states this project is licensed under GPLv3. Please follow instructions at http://www.gnu.org/licenses/gpl-howto.html
Mainly, you need to add a COPYING file with the full text of the GPLv3 license, and add a license header to each source code file. The header should contain this text: (replace year with the year[s] the project is/was active in, eg. 2011-2012, replace Full Name with your full name, and replace email with your email address)
Example copyright line as seen in one of my projects: Copyright (C) 2010-2011 Elad Alfassa elad@fedoraproject.org