Ring opening pliers come in a few different sizes. If you're working with tiny rings that are also small in gauge (18g, 16g, 12g) then getting a mini ring opening plier will probably be the easiest to use. The heavier the gauge of the ring, the larger the plier you will need. This is because you need leverage to be able to open up the ring and a small plier will not work well for large heavy gauge rings.
One thing to also note, is that you never want to apply large amounts of pressure when doing this. If you open a ring too much, it won't go back to its normal state. And that point you'll have to use a ring closing plier to help close it back.
To insert a bead into the ring:
To remove a bead from the ring: