Edit in JSFiddle

<div class="flex-container">
<div class="flex-item">flex item 1</div>
<div class="flex-item">flex item 2</div>
</div>

.flex-container {
    display: -webkit-flex;
    display: flex;
    -webkit-justify-content: center;
    justify-content: center;
    width: 300px;
    height: 240px;
    background-color: Silver;
}

.flex-item {
    background-color: DeepSkyBlue;
    width: 100px;
    height: 100px;
    margin: 5px;
}