Understand how e-tickets work. Somewhere on the ticket image will be a bar code. This bar code is essentially your ticket. The ticket-taker at the venue will scan the bar code and verify that the ticket is legitimate. Generally, each individual ticket needs its own bar code, so if you purchase two tickets to an event, your e-ticket will include two pages with separate ticket images --- you'll print out two pages, so your companion has his own ticket.
Download Adobe Reader from Adobe.com, if the program is not already installed on your computer. Most e-tickets will be delivered in a PDF file. Adobe Reader opens PDFs so you can read and print them. It is free and easy to install.
Purchase your e-tickets from your selected ticket service. Make sure that the vendor is a reputable ticket broker that issues a confirmation number and contact number. When in doubt, purchase e-tickets directly from the venue's box office --- most major arenas and concert halls allow online purchases through their official websites and offer e-ticketing.
Check your email. You'll be asked for your email address when you purchase your tickets, and once your purchase is complete you will receive an email with an attachment of the ticket images. Download the attachment and open up the file using Adobe Reader.
Print the attached file using your home printer. Use plain white office paper. Make sure the bar code is sharp --- the rest of the pictures or words on the tickets can be slightly distorted, but the bar code needs to be clean and legible. Be sure to print out all pages that contain bar codes. Remember that each ticket has a unique bar code, so don't just print multiple copies of one ticket; print out all pages of the attachment.
On the day of the event, simply bring your printed tickets to the venue door and present them to the ticket collector.