What is the step by step procedure of transfer from international terminal to domestic terminal at Newark Airport?

If you want to change your flight to go directly from EWR to BOS, I would contact United as soon as possible. Most likely, you will be subject to a change fee on top of any fare difference between the flights, which makes the cost unpalatable for most people.

If this is a one-way ticket, you can forfeit the EWR-ORD-BOS segments and take the train or whatever you prefer to save yourself the out-of-the-way connection. Do not do this if you are on a round trip ticket, however, as failure to show up for any flight will result in all subsequent flights on your ticket being canceled.

Before arrival

You should be able to retrieve boarding passes for all three of your segments (BOM-EWR, EWR-ORD, and ORD-BOS) when you check in.

Your bags should be tagged for your final destination (BOS) when you check them at BOM. Since you are on a connecting flight on the same airline, there is no reason why they would not do this for you.

The flight attendant will distribute traveler entry forms for you to complete prior to arrival. For most people, only the customs declaration form (6059B) is of interest; it provides basic information about who you are and what goods you are bringing into the country. It speeds up the process to have this form completed before you deplane, but should you misplace it, there should be extra copies available after you land.

Transfer at EWR

Immigration

After you land at EWR, all passengers will disembark the plane and proceed to passport control. Just follow the signs for international arrivals, and don't worry, it is impossible for you to accidentally leave without being processed.

You did not specify your nationality, but if we assume you are not a U.S. citizen or permanent resident and not otherwise enrolled in Global Entry or a compatible program, you will wait in line until called to a kiosk by Customs and Border Patrol. The CBP officer will scan your passport, ask you a few questions about your visit, and stamp you in.

It is hard to predict how long you will have to wait in line, between the countervailing forces of COVID-19 reduction in travel and holiday season increase in travel, but the CBP has an Airport Wait Times website you can consult.

Baggage Claim and Customs

After you clear immigration, you will proceed to a baggage claim area where you will collect all your bags, then proceed through Customs screening, where you will present your customs declaration form and where your bags will be scanned. They may want to conduct a hand search of the bags; this is not uncommon.

In the event you have any illegal or harmful goods (foods and agricultural goods will come under particular scrutiny) you will need to discard them, and in the event you have quantities of alcohol, tobacco, or other restricted items exceeding the personal exemptions, you will need to pay duty on them.

Baggage Re-Check

After you clear customs inspection, you should review your carry-on items and see if you have any restricted items that you need to stow in the checked bags, chiefly drinks and other liquids (e.g. duty-free fragrances), and likewise if you want to retrieve anything from the checked bags. This is the only opportunity you will have until your final destination.

Once done with that, there will be airline staff waiting just past the screening area to re-check your bags with the airline. This is inside the customs area—be sure you do not exit the secure area with your bags. If you do, you will need to take them all the way out to the ticket counter to hand them over to the airline again.

By the same token, at this point, you are indistinguishable from any internal passenger who started the day in New York or Los Angeles and not Mumbai. If you plan to abandon your EWR-ORD-BOS flights, you can take your bags with you and simply leave the airport.

If you want to take a COVID-19 screening and have time before your connection, find the XpresCheck station on level 3 (Monday through Friday, 10am – 6 pm), prices ranging from $75 to $200.

TSA Screening

After you have re-checked your bags, you will exit the international arrivals area and be "landside" at the airport at Terminal B. You should not need to check in again, assuming you received all your boarding passes in BOM.

In the event you missed your connection due to long lines or other delays during screening, go to the ticket counter and explain the situation, and they should put you on the next available flight.

Check the monitors for the gate assignment for your EWR-ORD flight—even if you had a gate assignment when you departed BOM, it may have changed. Take the free AirTrain tram to Terminal C or Terminal A if necessary, and proceed to the TSA screening checkpoint.

There will be another line to stand in. Have your boarding pass and identification ready for the TSA agent to inspect. Once you go through the scanner, you will be airside and can proceed to your gate.

Connection at ORD

United primarily uses Terminal 1 (concourses B and C) at O'Hare. After you arrive, check the monitors for the gate assignment for your Boston flight and proceed there. You will not claim your bags and you will not check in again with the airline.

Arrival at BOS

United uses Terminal B at Boston-Logan. After you arrive, the flight attendant will announce which baggage claim number to use, and after disembarking the plane, proceed there. If you miss the announcement, the baggage claim number will also be displayed on arrivals monitors throughout the terminal.

It is always possible that one or more bags will have been misdirected at the connection. If you are missing a bag, go to the baggage office and have them track the last location it was scanned. They will look for it and should deliver it to the airport or a nearby location.