This is what you need to do to enroll in an undergraduate CS class if you are an undergrad. (If you're a grad student, you should be at a different page.)
If you are an undergrad, follow these steps, stopping once you're in the class or these instructions tell you to wait for email from the UCLA Samueli Office of Academic and Student Affairs:
If the course is really important to you, try to enroll during your first enrollment pass. Note that many courses are restricted during the first pass to majors that require that specific course, so you may have to wait until second pass if you are not one of those majors.
Try to enroll during second pass. If you can't get into the course or on the wait list because it is full or because you're not of a major that allows you to enroll, fill out the Enrollment Consideration Request Form (and read why it is important to fill out that form). Do this no later than about a week after your second pass appointment. Wait for email from OASA; you may hear nothing until close to the start of the quarter.
If the quarter has started, the instructor may or may not circulate a signup list the first day to assess any remaining demand for the class. An undergrad who has not both filled out the Enrollment Consideration Request Form in a timely manner and shown up in class the first day is usually unlikely to get in.
If the course you are interested in is CS 31, then instead of using the Enrollment Consideration Request Form, use the CS 31 form. Don't waste a first pass enrollment opportunity on CS 31.
If the course you are interested in is CS 32 (in a quarter in which it is offered), then instead of using the Enrollment Consideration Request Form, use the CS 32 form. Don't waste a first pass enrollment opportunity on CS 32 (unless it's for Winter quarter and you really want the TR 4-5:50 section).
Requisites for CS courses are enforced. If you believe you have fulfilled the requisite through a course taken at another school, but MyUCLA doesn't yet recognize that, then follow the instructions under the Enrollment Policies and Deadlines heading, Prerequisites subheading in this FAQ.
Do not send a message to the instructor of the course requesting a PTE before the first day of classes. Let the process work.
As soon as you decide you don't want to be in a class you're already enrolled in, please drop it so that the spot will be available for someone else.
If you filled out the Enrollment Consideration Request Form and are anxiously awaiting a reply that won't come for weeks or months, be patient. What you don't see are all the behind-the-scenes activities: negotiating agreements to enlarge the class, arranging tricky room swaps to get a larger room, ensuring funding for more TA positions for additional sections, etc. We've generally been able to accommodate most people who fill out the Form in a timely manner; if you fill it out late or not at all, we won't know to count you when requesting a larger room, for example.
To reiterate that last point: Please don't be one of those silly students who never gets around to filling out the Form, and then just as classes start, asks to get into some full class. Such a student is usually told that if they had filled out the form in a timely manner, they'd already be in the class, but now it's too late to be able to accommodate them.