Before you start, you need to have sane installed, and you also need tesseract-ocr - both should be available in your distros repositories.

$ sudo apt-get install sane-utils tesseract-ocr

Next you need to find out what scanners you have available, and you do this with:

$ scanimage -L
device `v4l:/dev/video0' is a Noname Vimicro USB Camera (Altair) virtual device
device `plustek:libusb:004:002' is a Epson Perfection 1250/Photo flatbed scanner)

Obviously the latter is my scanner.

Assuming you have a working scanner, the following is a simple two liner to scan and OCR.

$ scanimage -d 'plustek:libusb:004:002' --mode Lineart \
--format tiff -x 215 -y 297 --resolution 200 > example.tif

And finally convert to text with tesseract:

$ tesseract /tmp/example.tif example

You should now have a file example.txt in your current directory, which you can open in any text editor.

Obviously this has limitations - it works for single-page A4 portrait typed documents - but it gives you the basics.

You could probably experiment with the resolution, 200 worked for me, so I didn’t bother trying anything else.  Traditionally the higher the resolution the better, but I seem to recall that tesseract works better on 300 and below.

On my Epson Perfection 1250 I found that I needed to add the sane switch --warmup-time 0 as otherwise it never finished warming up.

If you would prefer to OCR an existing PDF, which is another thing that I find myself doing from time to time, then first convert it to a tif:

$ convert -density 200 example.pdf -depth 8 /tmp/example.tif

And then run the above tesseract command.