<ul class="flow">
  <li class="flow_item"><span>STEP1</span>流れ1のテキスト</li>
  <li class="flow_item"><span>STEP2</span>流れ2のテキスト</li>
  <li class="flow_item"><span>STEP3</span>流れ3のテキスト</li>
</ul>

ul li{
  list-style:none;
}
.flow{
  max-width:400px;
}
.flow_item{
  margin-bottom:2em;
  padding:1em;
  background:#ebebeb;
  position:relative;
}
.flow_item span{
  margin-right:1em;
  font-weight:bold;
}

.flow_item:not(:last-of-type)::after{
  content:"";
  display:block;
  position: absolute;
  top: 100%;
  left: calc(50% - 20px);
  width: 0;
  height: 0;
  border-style: solid;
  border-width: 20px;
  border-color: #f2c200 transparent transparent transparent;
}