Not that hard really. My average for a 'Naylor' NAN is about 7 stems per letter, but that is white euro after it's had a drink for a couple of days to allow the petals to open a bit. Worst case you might add a few stems to this so you should be safe with 5x5. However, as it's your Nan i would go for something nicer. I bought 'Arctic Queen' this morning. it's not as heavy as euro and you might end up using 10 stems per letter, but it does need a drink to sort itself out. Of course this is just a guide and everyone fills a bit differently, using more or less. As this is your Nan i shouldn't think you are too worried about the economics. Normally you would look at the size of the larger heads and work out how its going to fill across the letter. you could come straight down the center and then a row either side, or just a double row for a larger head. You then need to look at how you go round the bend. The trick is to not push this one in too far as you will end up with a ridge rather than a smooth curve. You should then have another row underneath this and again, leave it a bit proud. Now you can judge where you will pin your ribbon or where the greenery line will be. doing it this way you avoid having to fill a small gap at the bottom with all those little buds. i tend to use the buds as 'buttons' along the edge of the letters to fill the small gap between the larger heads, but only if it's needed. it can add a bit more interest to the piece although it is time consuming.
Feature flowers...i normally do a larger one top left and a smaller version bottom right. You could do these straight into the letters but i always add a small piece of oasis for each feature as it gives you more scope with the stem angles. I like to add these bits before ribboning so i can anchor tape them to the letter for added security. If you've ever seen one fall off you realize how difficult it is to get it back on. I also add some glue to the legs of the frame before assembly just to be sure they wont come adrift with handling. You can just run the glue gun along the join. When soaking the letters, remember that it holds a lot of water, so ideally drop it into a bath or if you are soaking it with a hose, don't stop too soon.
I think that just about covers it.
Good luck and sorry to hear about your Nan.