Start by calculating the Customs Duty. If the value of your package (including what you paid for shipping) is less than £135, there's no duty to pay. Otherwise you need to go here
https://www.trade-tariff.service.gov.uk/chapters/85
select the relevant type of goods and look under 'Third country duty' to find the percentage rate that applies.
In the unlikely circumstance of your appliance (together with its associated shipping cost) being worth less than £15, there's no VAT to pay. Otherwise you need to add together the cost of the appliance, the cost of shipping and and Customs Duty you've calculated from my first paragraph. Then calculate 20% of the total as the amount of VAT to be paid.
If (as would seem to be extremely unlikely) your figures from both of the above paragraphs are zero, there's no customs examination fee to pay. Otherwise it's a flat fee of £12.
Add together the figures from each of the three paragraphs above to arrive at the total you will need to pay.