It's a tab at the top of the page. Doing so opens the Messages pop-up window. This is in the top-right corner of the pop-up window. Click the name of the follower you want to message. You can also type the name of a follower into the search box if you don't see the follower listed here.  You can select more than one follower here, but you must select at least one follower. Keep in mind that, while you can select people who aren't following you, sending a message to them will deliver the message to their "Requested" Messages folder instead of their inbox. It's in the bottom-right corner of the window. Type your message into the text box at the bottom of the window. As with tweets, you can add a photo or a GIF to your message, though you can't add both (or more than one) of them at the same time:   Photo — Click the mountain-shaped "Photo" icon, select a photo or video from your computer, and click Open.  GIF — Click the GIF button, search for a GIF to use, and click the GIF. It's to the right of the message. This will send your message to the recipient. A grey checkmark under your message means it was sent. A blue checkmark means it was viewed.

TL;DR in Chinese:
点击主页顶端的私信标签。 点击新私信。 选择一个关注者。 点击窗口右下角的下一步。 输入私信内容。 你也可以添加照片或GIF动图。 点击私信右边的发送按钮。