The key rules with applying for EIS as a Knowledge Intensive Company (KIC) are all found within our guide to KIC criteria.
Generally speaking, it's best not to apply as a KIC unless you absolutely need to rely on one of the benefits. (As you can always apply later).
If you do need to rely on KIC status, then it's best to attach a separate letter, detailing how you meet the requirements listed in our article above. You may need to attach financials, employee profiles, and patent records.