In a space with good ventilation, spray the finished painting with a fixative from a distance of 30cm using an even motion, moving from one side to another from a consistent distance.
Be sure to go past the edge and not stop before it. You will want to keep the spray moving so that you don't soak an area but give a light, even coat to the whole painting. Wait 5 minutes, turn the work 90 degrees and repeat. Then check to see if the pastel is fixed enough by gently touching your finger to an edge of pastel and seeing if it is affixed. If it smudges repeat again.
Applying more, but lighter coats will change the appearance of the work less than fewer, but heavier, soaking coats. Fixatives often make whites become transparent and darks become darker, so you may need to top up your highlights and accept that most of the painting is stabilised except for the whites, so the work should be framed behind glass.