Welcome to FPN antikva. We have a few members who can write Arabic here so you'll get good feedback.
GP - thanks for reminding me of that thread - it generated some interest for a while. Here it is for anyone interested: Learn to write Arabic.
antikva - the letter combinations can be a bit tricky. The typical progression of learning to write Arabic is to learn the individual letters, followed by each letter in the initial form joined to all remaining letters and finally to forming small words. This usually is enough to give one a good understanding of how the ligatures are formed. I'm afraid I cannot give you a general answer for the differing slant on 'daal' and 'alif' - the 'daal' looks similar to a '>' sign joined near the middle. If the top part is straightened, it will look like a 'raa'.
Riqa, although a calligraphic script, is a good foundation for Arabic handwriting. You might find this workbook useful in giving you an idea of how to join letters: Riqa workbook on Scribd.
Feel free to ask about specific joins if they are giving you trouble and I will be happy to help.