If you are holding an offer and want to use clearing, you will need to self release into clearing, which will decline all your offers.
Whether you self release into clearing, or are simply not holding any offers to start with, you will then need to phone any uni(s) offering the course you are interested in to see if they will give you a verbal offer for that course (this normally happens on A-level results day for most students, although if you already have your results because you are an IB student or a gap year applicant and the course is already showing in clearing, you can get in touch now with them). Once you have a verbal offer, you will need to go on UCAS and add your choice and then the uni will confirm it.
They go first come first served so if you are an A-level student getting your results this year and planning to go through clearing, be ready to get up early on results day to start calling unis as soon as their clearing lines open!
As above, if the course is currently available in Extra just decline your offer now and get ahead of things by applying now
Note as above, you may need to submit a new personal statement when you apply through clearing if it's a different course, so get started writing it early!