1 Mix acid at 8:1 mix spray on and leave to dwell for 10 mins.
2 Rinse with fresh cold water.
If improvement was made repeat.
3 If there is still a stain spray with Craftex Shift, leave to dwell for 10 - 15 mins
4 Rinse with fresh cold water
If there is no improvement, dry the area before considering repeating.
This should work depending severity of the stain and the type of carpet. If it doesn't it may be all you can do. There are other ways of using Craftex Shift (Sodium Metabisilphite) which will improve its effectiveness however there is a risk of removing color from the carpet.
Have Fun