Lets start with the basics. Know the correct address for the person you are sending to. Make your handwriting neat, and use a pen that stands out from the colour of the envelope. If you put a half-assed address it will get a half-assed delivery. Remember that councils are very unimaginative and will reuse road names, sometimes multiple times across a city ... this is why postcodes are important. Here is a random postcode search on the Royal Mail postcode finder:
Similarly, in the countryside, cottage names will be equally unimaginative and repetitive, so include the village name. (When I worked in Morton every village had a Beech cottage, Cotswold Cottage, Church Cottage; Oddinton has three cottages just called The Cottage.) If your recipient lives in a flat, don't forget the flat number as well as the house number ... a flat number without the house number will also be unlikely to arrive. (Sometimes you can get away with this if it is a student residence block that has a reception that holds all the mail, but ...)
Use the postcode. Please. (Pretty please.) They are an amazing invention. If a letter or packet has the correct postcode the chances are that I can get it to the right place if for some reason the rest of the address is defaced or missing. They do have their limits however. Some places get one all to themselves, but sometimes 100 houses or more may share the same one, so do not assume they can be used to track down the house of your auntie Gladys who you haven't written to since she was in hospital three years ago.
Put a return address on the back (everywhere else in the world seems to put it in the top corner, but here in the UK we use the back). Everything you put in the postal system needs a return address. I mean everything!
(Images here from the Royal Mail 'How to address your letter' page.)
(Disclaimer: this A to Z is not official Royal Mail advice, except by coincidence.)
Stay safe. Be kind. Send someone a real letter.