Start with?

When you have chosen your decor items, start with placing the large ones first. Also, the ones that fill the background will go first. For example, a framed art should be it.

Tall objects

Implementing tall objects is a nice idea to add some height. And the most important tip is to place them outwards. Flowers , sticks, vases should be added near the edges.

Layering

After you have put the basic things, just fill the gaps. Put smaller and visually interesting things. You can use same items on various positions, in different depths. It will add dimension , creating sophisticated design like a proffesional would do it.