2. The most primitive way

You don't need apt-ftparchive (1) or debarchiver (1) (which are not available on a potato system) to make your custom package apt-gettable.

2.1. Create package overview files

apt-get (8) needs package overview files Packages.gz (for binary packages) and Sources.gz (for source package). To create them, use dpkg-scanpackages (8) (for Packages.gz) and dpkg-scansources (8) (for Sources.gz).

You need the following files to create them:

Put all the files in the same directory. Note that you don't need .changes file.

dpkg-scanpackages needs the override file. It is a text file for overriding the Priority: and Section: fields of the control file. It has the following format (one line for one package):
<package> <priority> <section> [<maintainerinfo>]
Each field is separated by a white space. <package> is the name of the package. Replace <priority> and <section> with the same values in the control file, and omit <maintainerinfo>. You can add comments by beginning them with # .

If you don't care the Priority: and Section: fields, you can use /dev/null as the override file. Run dpkg-scanpackages in this way:
$ dpkg-scanpackages . /dev/null | gzip -9c > Packages.gz
dpkg-scanpackages outputs the uncompressed overview files for binary packages ("Packages", not "Packages.gz") to standard output. Compress it with gzip (1). In this case the packages will be marked as "Unclassified without a section" by dselect (8). If you want to specify the Priority: and Section: fields, replace /dev/null with the name of the real override file.

For source packages, use dpkg-scansources instead.
$ dpkg-scansources . /dev/null | gzip -9c > Sources.gz

2.2. Upload

Upload the packages, the files Packages.gz and Sources.gz in the same directory of your web server, say, http://your.web.server/debian-custom/ .

In the above example, the "apt lines" (the lines for /etc/apt/sources.list) are:
deb http://your.web.server/debian-custom/ ./
deb-src http://your.web.server/debian-custom/ ./
Note that the slash at the end of each line is necessary.

2.3. Example

I back-ported the packages debarchiver (a tool to sort debian packages and make them apt-gettable) and opalmod (debarchiver needs it) to a potato system. I wanted to make them apt-gettable.

First I put the packages and their sources to a new directory.
$ ls
debarchiver_0.0.12potato1.dsc      opalmod_0.1.4-1potato1.dsc
debarchiver_0.0.12potato1.tar.gz   opalmod_0.1.4-1potato1.tar.gz
debarchiver_0.0.12potato1_all.deb  opalmod_0.1.4-1potato1_all.deb

I cared the Priority: and Section: fields, so I wrote a override file "override.txt"
# dpkg-scanpackages override file
debarchiver optional devel
opalmod optional devel

I could run dpkg-scanpackages and dpkg-scansources manually, but I wrote a makefile for easier maintenance instead.
all: Packages.gz Sources.gz

Packages.gz: override.txt
        dpkg-scanpackages . override.txt | gzip -9c > Packages.gz

Sources.gz: override.txt
        dpkg-scansources . override.txt | gzip -9c > Sources.gz

.PHONY: clean

        -rm -f Packages.gz Sources.gz
(Don't cut and paste this --- you must use TAB for each command.)

Then I ran make (1) to create package overview files.
$ ls
Makefile                           opalmod_0.1.4-1potato1.dsc
debarchiver_0.0.12potato1.dsc      opalmod_0.1.4-1potato1.tar.gz
debarchiver_0.0.12potato1.tar.gz   opalmod_0.1.4-1potato1_all.deb
debarchiver_0.0.12potato1_all.deb  override.txt
$ make
dpkg-scanpackages . override.txt | gzip -9c > Packages.gz
 Wrote 2 entries to output Packages file.
dpkg-scansources . override.txt | gzip -9c > Sources.gz
$ ls
Makefile                          debarchiver_0.0.12potato1_all.deb
Packages.gz                       opalmod_0.1.4-1potato1.dsc
Sources.gz                        opalmod_0.1.4-1potato1.tar.gz
debarchiver_0.0.12potato1.dsc     opalmod_0.1.4-1potato1_all.deb
debarchiver_0.0.12potato1.tar.gz  override.txt

I uploaded all of those files except Makefile and override.txt to http://www.interq.or.jp/libra/oohara/debian-unofficial/. Now you can download them with apt-get (8) if you add these lines to your /etc/apt/sources.list :
deb http://www.interq.or.jp/libra/oohara/debian-unofficial/ ./
deb-src http://www.interq.or.jp/libra/oohara/debian-unofficial/ ./