How do you say

I’ve reserved a flight online, and I’m flying to Beijing at the beginning of next month.

..in Chinese?

我在网上订了机票,下个月初会坐飞机去北京。

wǒ zài wǎng shàng dìng le jī piào , xià gè yuè chū huì zuò fēi jī qù běi jīng 。

How do you say "I booked train tickets three months in advance online." in Chinese?