Hello Everyone, I haven't really worked with creating a test class yet but I'd like to know how to write a test class for the following Apex class. If you could guide on how to write a test class that'd be awesome.

public class OptOutPreference {
    public static void OptOutPreferenceLink(){
        //Now these encryption keys are obtained via custom metadata, updated API version as well
        List<SFMC_Encrypt_Setting__mdt> encrypt_settings = SFMC_Encrypt_Setting__mdt.getAll().values();
        SFMC_Encrypt_Setting__mdt setting = encrypt_settings[0];
        String initializationvector = setting.InitVector__c;
        String base64key = setting.Base64Key__c;
        String encrypttype = setting.EncryptType__c;
        
        Blob exampleIv = Blob.valueOf(initializationvector);
        Blob key = Blob.valueOf(base64key);
        
        String bxurl = 'http://cloud.mail.buxomcosmetics.com/bx_unsubscribe?&EA=';
        String bmurl = 'http://cloud.email.bareminerals.com/bm_unsubscribe?&EA=';
        String lmurl = 'http://cloud.email.lauramercier.com/AuRevoirPage?&EA=';
        String cpburl = 'http://cloud.email.cledepeaubeauteus.com/cpb_unsubscribe?&EA=';
        String shurl = 'http://cloud.email.shiseidousa.com/shi_unsubscribe?EA=';
        String narsurl = 'http://cloud.email.narscosmetics.com/nars_unsubscribe?&EA=';
        String narscaurl = 'http://cloud.email.narscosmetics.ca/en_ca_nars_unsubscribe?&EA=';
        String shcaurl = 'http://cloud.email.shiseido.ca/en_ca_sh_unsubscribe?&EA=';
        String cpbcaurl = 'http://cloud.email.cledepeaubeaute.ca/en_ca_cpb_unsubscribe?&EA=';
        String lmcaurl = 'http://cloud.email.lauramercier.ca/en_ca_lm_unsubscribe?&EA=';
        String deurl = 'https://cloud.cheers.drunkelephant.com/unsubscribe?EA=';
        String decaurl = 'https://cloud.hello.drunkelephant.ca/unsubscribe?EA=' ;
        
        List<Account> OptOutEmail =[SELECT Id, PersonEmail, Brand__c, CreatedDate, Opt_Out_Preference__c FROM Account WHERE CreatedDate < 2022-09-15T16:00:53.000+0000 AND PersonEmail !=NULL AND Brand__c !=NULL AND Opt_Out_Preference__c = NULL];
        System.debug('AJ' + OptOutEmail);
        List<Account> UpdateAccounts = new List<Account>();
        
        for (Account a : OptOutEmail){
            Blob data = Blob.valueOf(a.PersonEmail);
            Blob encrypted = Crypto.encrypt(encrypttype, key, exampleIv, data);
            String s = EncodingUtil.urlEncode(EncodingUtil.base64Encode(encrypted),'UTF-8');
            if (a.Brand__c == 'Buxom')
            {
                a.Opt_Out_Preference__c = bxurl + s;
            }
            if (a.Brand__c == 'bareMinerals')
            {
                a.Opt_Out_Preference__c = bmurl + s;
            }
            if (a.Brand__c == 'NARS')
            {
                a.Opt_Out_Preference__c = narsurl + s;
            }
            if (a.Brand__c == 'Laura Mercier')
            {
                a.Opt_Out_Preference__c = lmurl + s;
            }
            if (a.Brand__c == 'Shiseido')
            {
                a.Opt_Out_Preference__c = shurl + s;
            }
            if (a.Brand__c == 'Cle de Peau Beaute')
            {
                a.Opt_Out_Preference__c = cpburl + s;
            }
            if (a.Brand__c == 'NARS CA')
            {
                a.Opt_Out_Preference__c = narscaurl + s;
            }
            if (a.Brand__c == 'Laura Mercier CA')
            {
                a.Opt_Out_Preference__c = lmcaurl + s;
            }
            if (a.Brand__c == 'Shiseido CA')
            {
                a.Opt_Out_Preference__c = shcaurl + s;
            }
            if (a.Brand__c == 'Cle de Peau Beaute CA')
            {
                a.Opt_Out_Preference__c = cpbcaurl + s;
            }
            if (a.Brand__c == 'Drunk Elephant')
            {
                a.Opt_Out_Preference__c = deurl + s;
            }
            if (a.Brand__c == 'Drunk Elephant CA')
            {
                a.Opt_Out_Preference__c = decaurl + s;
            }
            UpdateAccounts.add(a);
        }
        update UpdateAccounts;
    }
    
}