Print the email you received from them, fill it up, arrange required documents (photocopy of passport, marksheets, degrees etc.), send all these along with printed-filled form to ICAS. Mention file number on the envelope.
Request transcripts from the universities by sending them the filled ICAS academic request form. Send a separate form for each degree even if the university is the same. Make the university courier the transcripts to ICAS with the envelope bearing the file number.
After everything is received by ICAS, wait for 40 weeks. While waiting, follow this
thread about ICAS.