Visit WechatPay (International business click here) website and register your account. you will need to input your email address and then verify it, setup your login password and pay password. You'll get Mchid and Appid in the open mail.
Login WechatPay set your Apikey.(在微信商户平台(pay.weixin.qq.com)-->账户中心-->账户设置-->API安全-->密钥设置中).
You will need to prepare WechatPay Mchid , Appid and Apikey.
Input your MchId , AppId and Apikey in Wechat and choose the country and currency when you apply, Save .
Then visit your Shopify Store background, Input your Shopify Gateway key and Gateway Password in Shopify Store background and Save.
(If you don't have WeChat payment gateway, click here to install)