_{I am not a lawyer. You should get one, if you are concerned about possible legal issues.}

You have to include a copyright notice (a ‘manifesto’, if you will) to each file that contains your work.

That ‘manifesto’ shall consist of:

• A line Copyright © <years> <owner>.

  Where <years> are years when this very file was created and substantially modified; you’ve said that you did not work on the project for a long time, so it might look like 2008, 2015 for example; do not use range (2008–2015) unless the file was modified every year.

  <owner> is who holds exclusive rights on this very file. The authors are owners if they did not transfer rights to someone else. An employee typically transfer his copyright on the work his paid for to an employer by terms of contract or (in many countries) by the law. So if you was working on the project during your work hours, you would better check that your employer does not claim copyright on it.

• Either ⓐ the full text of a license in the case of short permissive licenses, e. g.: BSD licenses (2 clause, 3 clause), X11, Expat, zlib licenses; or ⓑ something that refers to the full text: Apache License v2 (see under APPENDIX), GNU licenses (see under ‘How to apply…’ in the GNU GPL) and other long and well-written free/libre licenses; also WTFPL :-), which is a sort of parody on complicated copyright licenses.

  When ⓑ you have to put the full legal text of a license in the file COPYING or LICENSE in the root of code repository usually; when ⓐ this in optional and does not make any sense from a legal point of view, but often considered a good habit since it helps to detect the license automatically, especially when a code hosting service does not have, like GitHub, a field for license in project properties.

You probably would also want to mention copyright terms briefly in README file.

I also have to note with regret, that many amateur free software developers neglects these legal technicalities, please do not take them as an example.

The standard way to apply a license to a software is to put the full license text in a file called LICENSE or COPYING included with the source code and then put a short notice in a comment at the top of each source file (naming the copyright date, holder, license and saying where to find the full text of the license).

GPLv3 requires that if the program does terminal interaction, it also has to output a short notice like this when it starts in an interactive mode:

Copyright (C) <year> <name of author>

This program comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’.
This is free software, and you are welcome to redistribute it
under certain conditions; type ‘show c’ for details.

If the license you want to apply doesn’t have specific instructions (e.g. zlib) OSI suggests to follow the Apache License / GPLv3 instructions.

According to Github a LICENSE file in the root of the repository is enough (see Where does the license live on my repository? and I already have a project with a license file, do I need to do anything?).

Further details:

• Applying the Apache License, Version 2.0
  

  To apply the ALv2 to a new software distribution, include one copy of the license text by copying the file:

  http://www.apache.org/licenses/LICENSE-2.0.txt

  into a file called LICENSE in the top directory of your distribution.

  […]

  Each original source document (code and documentation, but excluding the
  LICENSE and NOTICE files) SHOULD include a short license header
  at the top.

• How to Apply GPL v3 Terms to Your New Programs
• I want to use the Mozilla Public License for software that I have written. What do I have to do?
  

  To apply the Mozilla Public License to software that you have
  written, add the header from Exhibit A of the license to each source
  code file in your project. Sample headers for various commenting
  styles are available here.
  You may also add additional accurate notices of copyright ownership, such as the name of the copyright holder, but this is not necessary.

This answer is for reference purposes only and it is not intended as nor does it constitute legal advice.
The author is not an attorney and makes absolutely no claim to have any knowledge about legal matters beyond that of an informed layman.