I'd add the salt in a few (maybe 3 or 4) doses over a day or two. Never add salt grains directly to your tank; always take some tank water out, dissolve the salt in it, and then re-add the water. Or, if you happen to be doing a pwc
, add the salt to the new water you are going to be adding to the tank.
Also, note that as you are raising temp you are gonna be speeding up the life cycle of the ich parasite, which means that for a few days it is going to be looking like the ich is getting worse. Don't give up! I started heat treatment on my female betta with ich when I saw *two* spots. About 4-5 days later my betta was literally covered with ich spots--she looked horrible! That was a day or so ago. As of right now, she is down to only 1 spot left.
For those who don't really care about biology, you can stop reading now. But for those who do, basically what is happening with heat treatment is NOT that the heat "kills" the ich...an ich parasite itself can survive quite fine above 86*F. However, ich cannot *reproduce* at 86*F or higher. So that is why heat treatment works, but also why it always look like the ich is getting worse before it gets better. As the ich already in your tank is exposed to increasingly higher temperatures, it moves through its lifecycle faster...which means at first, more and more ich is attaching itself to your fish. However, none of these ich parasites are able to reproduce...so once all the ones currently alive in your tank die off, the infestation is ended.
And as for salt, technically speaking it is not needed at all as long as you keep up the heat. But it is helpful for a couple of reasons already mentioned by others--since high temp water holds less oxygen, a little salt in the water is said to help gill functioning and so the fish can better "breathe" the oxygen in the water. Plus, since each of those ich spots when it finally detaches forms a small lesion in the skin (probably too small to see with the naked eye, but a lesion nonetheless), some salt in the water acts as a general tonic in order to help ward off secondary infections (bacterial or fungal) developing at the sites of those ich wounds.
Last in the realm of semi-useless information, even without heat at all, just a high enough concentration of salt can kill ich by itself, even at room temperature. However, to treat with salt only you are talking about a concentration of around 2-3 ppm
...I'm not exactly sure what that translates to in terms of teaspoons per gallon, but it is reasonably high.