Here you go.
$('#PrincipalSignature_sketch').sketch();
$("#PrincipalSignature_clear").bind("click", function () {
var canvas = document.getElementById('PrincipalSignature_sketch');
var context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
context.beginPath();
});
<canvas id="PrincipalSignature_sketch" width="300" height="100" style=" border: 1px solid #ccc;"></canvas>
<button type="button" id="PrincipalSignature_clear" class="btn btn-default btn-sm btn-clear-canvas" data-tool="eraser">Clear</button>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script src="http://intridea.github.io/sketch.js/lib/sketch.min.js" type="text/javascript"></script>