The online Heathrow consultation can be reached here:
https://afo.heathrowconsultation.com/
Submissions must be made by 4 March 2019.
A guide to the consultation prepared by FohSoc for SE Londoners can be accessed here:
https://1drv.ms/p/s!AuV6NOp8I8tAkX-wJntuswM5961T
Obviously none of the answers is mandatory and you may wish to raise points not included in the guide.