How can I position one element below another?

just give position : relative to second div and top:315px or what ever you want

.first{
     width:70%;
     height:300px;
     position:absolute;
     border:1px solid red;
 }
.second{
    border:2px solid blue;
    width:40%;
    height:200px;
	position: relative;
    top: 315px;
}
<html>
<head>

</head>
<body>
<div class="first"></div>
<div class="second"></div>
</body>
</head>

Here is a solution:

.first{
     width:70%;
     height:300px;
     position:absolute;
     border:1px solid red;
     box-sizing: border-box;
}
.second{
    position: relative;
    border:2px solid blue;
    width:40%;
    height:200px;
    top: 300px;
    box-sizing: border-box;
}
<div class="first"></div>
    <div class="second"></div>

And you can to not point position, because div is block element and will be placed at new line by default.

.first{
     width:70%;
     height:300px;
     border:1px solid red;
 }
.second{
    border:2px solid blue;
    width:40%;
    height:200px;
}
<div class="first"></div>
<div class="second"></div>