Hi Nède,
I'm still a beginner but I've been doing this with some success. Many of the hints and tips you will see for paper negatives are intended to reduce contrast. These include pre-flashing, using dilute developer, and using a yellow filter with variable contrast paper. For salt prints, just skip all of that, use your photo paper straight out of the box, and give a bit more exposure than you would to make a normal contact print. Develop to completion with full strength developer. It is okay if the highlights are very dense!
Once you have your negative, a starting point for exposure in your contact printing frame is about an hour or 90 minutes facing open sky to the North ( away from the sun ) followed by about 20 or 30 minutes direct exposure to the sun... although it depends on the time of day and time of year quite a bit. So far I've been finding something different from what I've read, which is that the full sun exposure seems to bring out contrast in the shadows and midtones while the north facing exposure seems to bring out detail in the highlights. At least, that's my impression so far...
I have a single BLB fluorescent bulb and it is difficult to make a salt print from a paper negative with it. Starting time is about 14 hours, up to about 16 or 20 hours and one problem is that the salt print can start to develop some fog in that amount of time. If you are working with exposures that long, it helps to add a little more citric acid... I'm using 6% and the paper doesn't start to fog until about 24 hours after coating. Also it does not develop as much contrast as prints made in the sun.
Salt prints can be very delicate and beautiful, and they can show very lovely subtle detail in the highlights. They are so pretty when they come out of the contact frame, it can break your heart when they change in the hypo.
Have fun, and be patient! It can take a fair amount of playing around to start to get the details right, so remember to have fun doing it and don't get too set on the idea of having it look a particular way until you've done a few and have a feel for what they can be like.
I'm going out tomorrow to make two paper negatives for salt printing, if either one comes out you'll see it in the MSA gallery soon!
Good luck and have fun with the paper negatives!