https://www.parktool.com/blog/repair...railleur-bikes has all the information you need. To summarize, if the chain length was good before you can simply copy it. With master link chains you simply cut the chain to length so that it is short one set of outer plates, and then insert the master link plates into the inner plates of the chain, link them together, and then pull the chain to lock them into place. This actually makes installation easier/harder to mess up. They're reusable if you ever need to remove your chain (for example, to clean it thoroughly, or install a new derailleur). There's a special tool that makes this very easy, but you can do it by hand usually by forcing the plates together--you can also try various tricks using wire/etc.

If your chain was not worn it's probably not lack of proper lubrication that caused it to fail. Probably it was installed poorly or experienced some trauma, and your massive watts were simply too much for it. That's great news, now you're only <$20 down the hole in consumable costs and have a useful tool.

This is a case where a shop may have actually saved you some money--on a broken but unworn chain I am often happy to install an inexpensive quick link with $5 labor when I'm reasonably confident the rest of the chain is fine.