The amount is a matter of personal preference, which is typical for a loose bath salt: you control the strength of both the mineral feel and the pine aroma. For a standard bathtub, a reasonable starting point is two to four tablespoons. If you prefer a more noticeable sensory effect, you can increase the amount the next time; if you are new to scented mineral baths, start with the smaller amount. If the label on your jar gives a specific recommendation, follow that first.
Add the salt to warm water while the tap is running, and stir the water briefly with your hand. Warm, not very hot, water helps the salt dissolve quickly and keeps the pine scent pleasant rather than sharp. Sprinkle the salt over a wide area rather than pouring it all into one spot, so it does not settle as a thick layer on the bottom. The salt dissolves easily, and the water becomes slightly mineral-rich, which can leave the skin feeling softer and less rough after bathing.
There is no need to rinse off afterwards, although a quick fresh-water rinse is a fine option if you prefer not to carry the scent with you or if your skin feels sensitive. How long to stay in the bath depends on your own comfort; ending while the water is still comfortably warm usually feels better than waiting until it cools. After bathing, pat the skin dry and moisturise if that is part of your usual routine. Keep the jar closed in a dry place, because sea salt absorbs moisture and the pine scent can fade if the container is left open.