Metal keyrings are some of the easiest items to produce as they are just pressing onto a small metal plate that is then fixed to the main metal keyring body.
Please ensure to remove the protective film on the insert before applying the sublimation paper and pressing it otherwise you will not get a good result.
To save yourself time try pressing multiple inserts at the same time. If doing this ensure you keep them as much in the centre of the pressing area of your heat press to ensure they all get the same amount of pressure.
Face the inserts face down so you are heating the metal first. This helps to prevent burning the paper.