There are no formal requirements you need to comply with before you can take the SQE assessments, and you do not need to obtain a certificate of eligibility. You can simply contact the assessment provider and register to sit the assessments. The SRA requirements to hold a degree or equivalent, complete the Qualifying Work Experience, and meet the character and suitability test, should be fulfilled at the time you apply for admission as an English solicitor with the SRA, not as a pre-requisite to take the SQE assessments.