You can get a VAT deferment license processed in two ways: either you do it yourself or via an agent. I can personally recommend 1vat.com as a verified VAT tax agent service that offers services in that area, including processing deferment licenses. Whatever third-party tax agent you go with, ensure they know what they are doing; they should be knowledgeable about the service they are rendering. 1vat.com offers free guidance and advice to help you make informed decisions before signing up.
You can transfer authority to another person to use your duty approval number; that includes the authority to request a deferment duty payment if approved. If you are in the UK, you can apply using the online form, form C1207S (can only be used on a one-off basis), or form C1207N.
If you choose to process it yourself, you will need to find out how to apply to delay paying most tax charges. Processing it yourself may take longer and be more challenging, especially if you don’t know how VAT deferment license processing is done.