Closed coded rings are available from The Budgerigar Society and The Budgerigar Club. The Budgerigar Society include Area Society rings and are necessary if you want to exhibit your birds at shows in young bird classes. They show the unique breeder number, a sequential number and the year. They are also a different colour each year so that the age of the bird may be easily determined.
Plastic split rings can be put on at any age, are easy to put on and remove and come in a variety of plain and striped colours. They are often used for quick identification of birds in the same family. See video https://www.youtube.com/watch?v=51cRBNxL5ig
The ring may be placed on either leg and to do so the chick’s leg should be held between thumb and forefinger, support the chick in your hand and group the three largest toes together and slide the ring on. The ring should be positioned up the leg and the back toe will be trapped underneath it, with either a ringing tool or sharpened matchstick this toe can be released. Initially, this process is not as easy as it sounds but soon becomes second nature.