<ol class="cp_clist">
  <li>css</li>
  <li>html</li>
  <li>copy</li>
  <li>easy</li>
</ol>

ol.cp_clist {
  padding: 0;
  counter-reset:cp_clist; /*数字をリセット*/
  list-style-type: none;
}
ol.cp_clist li {
  position:relative;
  background: #F8BBD0;
  padding: 0 5px;
  margin: 0 0 10px 30px;
}
ol.cp_clist li:before {
  position: absolute;
  display: flex;
  justify-content: center;
  align-items: center;
  /* カウントさせる */
  counter-increment: cp_clist;
  content: counter(cp_clist);
  /*装飾*/
  font-weight: bold;
  color: #fff;
  background: #F06292;
  width: 20px;
  height: 20px;
  text-align: center;
  /*位置の調整*/
  padding: 0;
  margin: 2px 0 0 -30px;
}
ol.cp_clist li:after {
  position: absolute;
  top: 9px;
  left: -7px;
  content:'';
  border-top: 4px solid transparent;
  border-bottom: 4px solid transparent;
  border-left: 7px solid #F06292;
}