Welcome to BenzoBuddies.
As [...] suggested, it would be useful to know for how long you have been taking Temazepam. According to Ashton's figures, 15mg Temazepam is equivalent to about 7.5mg of Valium.
Is the Restoril in tablet forum? Either way, it will be better to take the Restoril for a short while (at the same dose as the Temazepam - 15mg per day, according to your post). This will allow you you to adjust to any difference that might exist in actual dosage between the generic Temazepam and brand Restoril (actual dosage can vary between manufacturers). You should then switch out as small a part of your dose as possible for an equivalent dose of Valium. If the Restoril is in capsules form too, you will probably find it difficult to split them, so you will have to switch one whole capsule (7.5mg) Restoril for 3.75mg Valium (three-quarters of a 5mg tablet). I'd recommend that you stick at this dose of of 7.5mg Restoril and 3.75mg Valium for at least two weeks before switching your second tablet - this will allow you time to adjust. Once you have switched out your second tablet, again stick at this dose for a couple of weeks, or longer, if you feel you have still not adjusted. At this stage we can then discuss your taper. It seems a little premature to do that now, but it is pretty straightforward.
If your Restoril comes in tablet form, then this will allow you to make smaller cuts, and you can split the pills in half - or even better, quarters. This doesn't have to mean a very drawn out period for the switch to Valium, as the smaller the amount you switch, the less time you are likely to need to adjust before making another switch. There is a complication though - your 5mg Valium tablets don't easily divide into amounts that are equivalent to half of a 7.5mg Temazepam tablet. Half of a 7.5mg Restoril tablet is equivalent to about three-eighths of a 5mg Valium tablet. It is unlikely that you will be able to split the Valium tablets into eighths, so you have to compromise. I've been thinking about what you could do, and to me, the most straightforward way would be to exchange half of the 7.5mg Restoril tablet for half of the 5mg Valium tablet (2.5mg). This is a slight increase of your overall benzo dose, but in reality it takes time for the Valium to build up in blood stream (whereas your blood levels of Restoril will drop quite quickly), so it should equal out to some extent. Once you feel OK with the switch (perhaps a week), then switch the second half of the Restoril pill for just a quarter of a 5mg Valium tablet (1.25mg). Again, allow about a week (or longer, if needed) for you to adjust to the switch. These two switches will add up to a reduction of 7.5mg Restoril (one 7.5mg tablet) and and uptake of a 3.75mg Valium (three-quarters of a 5mg Tablet). You will ten go through the same process with your second Restoril pill. By the end of the switchover, you should be taking 7.5mg Valium (one-and-a-half 5mg tablets) per day.
I also strongly suggest that you obtain 2mg tablets of Valium, as they will allow you to make smaller cuts after you have completed the switch. I don't think that there is much point in obtaining 2mg tablets of Valium for the purposes of the switch, as these tablets don't divide into anything that is convenient for the substitution either. Although splitting the Restoril tables may seem a bit complicated, I do think it is preferable to switching a whole tablet at a time.
As I wrote earlier, we can discuss the taper after you have finished switching. You are going to need far more then the fifteen tablets of 5mg Valium (you will need a months supply, maybe more), and don't forget the prescription for the 2mg Valium tablets.
Take care, and good luck.