<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;
  margin: 0 0 0 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*/
  background: #03A9F4;
  color: #fff;
  border-radius: 50%;
  width: 20px;
  height: 20px;
  /*位置の調整*/
  padding: 0;
  margin: 2px 0 0 -30px;
}
ol.cp_clist li::after {
  position: absolute;
  top: 7px;
  left: -15px;
  content: '';
  border-top: 5px solid transparent;
  border-bottom: 5px solid transparent;
  border-left: 10px solid #03A9F4;
}