2) Apply via Disclosure Scotland : https://www.mygov.scot/basic-disclosure/
3) Apply via AccessNI : https://www.nidirect.gov.uk/campaigns/accessni-criminal-record-checks
It doesn't matter which one you do. All three ways to apply are for the same basic disclosure certificate.
If you already have the more detailed standard or enhanced disclosure certificate via another employer from other work you do, please read this