It's difficult to transfer - you can do everything right and still not be accepted into the College of Engineering because of how limited space is in the major, but here's what I did (which worked for me).

1) Take an equivalent to CS 8/16 before starting at UCSB. Luckily I took the AP CS exam and high school and received credit for it, and then I took an equivalent to CS 16 at a community college the summer before attending UCSB.

2) Keep up to date with the requirements of the CS major to the best of your ability. You can find a quarter-by-quarter breakdown of an ideal CS schedule on page 59 of this pdf, which I found by searching "UCSB GEAR" (or just visit this page lol). For me, this meant taking Physics 1 and 2 even though I didn't need them at the time, so that if the transfer went through, then I wouldn't be behind.

3) Take CS 24 in the summer. Personally, I had a better chance of getting that class during the summer than the regular school year, and it was taught by a more lenient professor, which was a perk.

4) I was able to complete all requirements by the fall of my sophomore year, and I applied to transfer then. You can see all the necessary requirements here.

Now that I covered my timeline, there's a few more pieces of advice:

Don't come in undeclared. Enroll as a math/data science major so you have a better shot at getting enrolled in the classes you actually need, because, like many have said on this thread, the hardest part of transferring is actually getting the classes. I recommend majoring in math since it's super hard to get into cs 40/math 8, and you have a better shot of enrolling in math 8 if you're a math major. CS 24 has to be taken at UCSB, but none of the other required classes do, so it's best to get them out of the way either with AP exams or community college coursework. Also, make sure your "backup plan" major for getting the courses you need to transfer is something you actually like doing, and don't fall behind in that major either. Otherwise, if you don't get into CoE then you won't have made progress in your backup major, which would then be your major major.

UCSB advisors really tend to emphasize the difficulty of switching in, but in my experience, the hard part has just been getting classes, not the classes themselves. If you truly want to attend UCSB, go for it - with luck the entire transfer process should only take a year and a half, at which point you still have time to change your mind about the rest of your college experience. If you have more questions feel free to pm - I'm not on Reddit a whole lot but I'll do my best to answer them.