This is the basic step of Applying to US Universities. You need to fill up a form. Forms are now online. However every online application has some offline component.
Online Application Form: Every University has its own set of questions and one should pay attention to instructions. You should try to save your work after completing every page or form. Try to submit the online form at least one week before the deadline, as the University servers get slow or are even down in the last few days due to heavy traffic.
Online Letters of Recommendation: Some Universities allow the letters of recommendation to be submitted online. In other cases you need to supply the names of the people who will be sending the letters of recommendation offline.
Online payment of fee via credit cards: Credit card payment is the preferred mode of payment. The credit card should be accepted internationally.
Transcripts: Transcript is a collection of all your marksheets with a covering letter from your University explaining the grading system. If students from your University or College have applied to US Universities in the past then the registrar's office would be familiar with the process. Start your attempts to get transcripts many months in advance.
Letters of Recommendation: Each University has its own rules and format for the letters of recommendation. The recommender needs to fill answer a few short questions, grade the applicant in the format provided and attach additional information. The letter must be sealed and signed across the envelope flaps. Some Universities require that the letters be posted independently. If not, then you may enclose the letters of recommendation in your offline packet with the transcripts, financial papers and other offline material.
Supporting Documents: Read the instructions carefully regarding what other supporting documents are required. Even if not mentioned, include a photocopy of all your score reports like GRE/GMAT/TOEFL. Any document related to your extra curricular document should not be sent. Any document till the high school level should not be sent.
Financial Documents: Even if it is mentioned as a requirement, financial documents are not required for the admission decision. They come into the picture once you are accepted and the University needs to process your I-20.
Submit online application earlier than offline packet
Even after sending all online and offline materials, a key component remains - you need to request ETS to directly report your GRE/GMAT/TOEFL/ other relevant test to your target Universities. There are no exceptions to this. However you should include a photocopy in your offline packet.
It is your responsibility to track with ETS and with your target University whether your scores were received. You should request ETS at least 30 days before the application deadline.