HSMP Approved - Timings of EC

[curious canadian]

Currently I am working in the US. I have a query about EC.
I got my HSMP approved. I was so absorbed in HSMP processing, I didn't think of EC before hand, and now that stage has come.
Can someone please look into my question - I am new to this - I really need good advice.

I applied for HSMP, so that 2 companies I am already in touch with could pursue my candidacy and I can apply for more UK jobs.

The company in UK, which I have been in touch with, wants to bring me in for a face to face interview - can I travel to UK after HSMP approval or should I first get EC approved and then go for my interview to UK?

1/ Can someone please advise me on the timing of EC - when it should be applied for - in my case?

2/ Can some please tell me how to prepare for EC - what do I need for EC?

Thanks a lot guys/gals for your help here - I really appreciate it.
Can you please look into this request - Thanks again in advance!