Hi Chef,
To begin with tap on the Shop icon at the left bottom of the screen to open Shop UI. Under Backyard Tab select the Tree you want to plant and drag it to the Backyard. Once the placement is done, tap on the green tick mark to complete planting the Tree.
Once successfully placed the Apple will be ready to harvest after a specified time. You can always watch an Ad or spend Cash to speed up the harvest.
Please note that a new Apple Tree can be harvested 3 times. After this Tree will have to be watered or revived by seeking help from your Friends.