Which program are you using?

I've never designed an Arabic font, but as far as I know you can use Arabic OpenType feature tags like 'isol', 'fina', 'medi', 'init', etc to help your substitution.

Hope this links help:
http://29letters.wordpress.com/2007/05/15/generating-arabic-fonts/
http://typophile.com/node/16288

Which program are you using?

I'm using Fontlab and MS VOLT. I prefer doing all the OT related work in VOLT.

الخط هندسة روحانية ظهرت بآلة جسمانية

Dear معز

These Microsoft tutorials will be a good starting point.
Developing OpenType Fonts for Arabic Script (1 of 3): Introduction
http://www.microsoft.com/typography/OpenType%20Dev/arabic/intro.mspx

and

Creating and supporting OpenType fonts for the Arabic script
http://www.microsoft.com/typography/otfntdev/arabicot/default.htm

You can also download some freely available (with open VOLT project) fonts from
http://www.crulp.org/software/localization.htm

Best regards,
Atif

You will need two separate lookups for this, and you will need to apply them in sequence. The normal layout feature to use for such lookups with be Contextual Alternates 'calt'.

Lookup 1, using VOLT syntax, will be like this

be -> be.alt

with the context string

| noon

Lookup 2 will be like this

noon -> noon.alt

with the context string

be.alt |

Note that you can foolproof the first lookup by putting both noon forms in the context like this:

| {noon, noon.alt}