Stickers are easy to use! But there is a main nuance - after installing stickers from the store designed specifically for installing applications for imessage, you will not see the stickers as a separate application installed on your phone.
To use the stickers, you need to open a new correspondence in iMessage or open an old correspondence at the bottom, or above the opened keyboard, you will see a panel with applications offered to you for use in iMessage.
Newly installed stickers can be located in the invisible part of this panel, so to use them you will first need to find them by flashing the application icons from right to left.
To do this, in the new iMessage dialog or in the existing one, you need to scroll right to left to the end with the proposed applications and click the rightmost icon with three dots ... (More).
If you do it right, you will get your favorites by iMessage selected stickers and will always be displayed on the left side of the panel in messages.