How to move stack from inventory Xbox one sims 4

Tap X / (Square for ps4) when in your sims inventory to grab the item out of it. Then hover over the sim or object you want to place the item in and tap A / (x ps4)  

So to plant a flower in the garden you tap X to grab it out of your inventory, then place it on the ground where you want to plant it. Then hover the cursor over the plant and direct the sim to plant the the flower. (you can then move the plant around in build mode if you don't get it exactly where you wanted to plant it.)