You will need a steady hand and spray backwards and forwards across your work slowly. Past the edges, but don’t ever make a stop in the middle. The aim is to keep your arm moving, but you also only want a fine spray. It is horrible if the spray sticks somewhere and ends up dripping all the way down, most probably giving you nasty dark patches.
So now what happens? You have decided you want your painting to keep the bright colours, and yes, you really want to fix it. My suggestion would be to spray it on as you paint various parts. This needs to be done before you put on any last layer of colour or do any retouching. And this way it should keep its freshness.
Otherwise you are likely to find that you are disheartened by the fact that your work seems to be ruined by a dull effect. I also find that if you apply it this way, you will not have problems with new colours mixing amongst ones applied earlier.
To get the best and freshest colour result leave the final colour layer unfixed. The choice is yours, but like I said it is never easy!