Know the hardware you are using for your router, really, really well...before undertaking such a thing. With the provided instructions it won't be too big of a deal to do the compiling. But that is only half the battle. Getting the compiled result to properly talk to the hardware in your router...that is where most fail.
So be sure you know the hardware really well.
Perhaps it is smarter to have a look at a working setup of the Tomato router setup and how it is linked to that particular hardware. That can give you a lot of insight, before embarking on your custom build.
Then there is the question, why custom? From (bad) memory, Tomato is not that actively developed anymore. OpenWRT is. After cleaning up a "junkyard" of computer gear I found several old and broken TP-Link WiFi routers (TL-WR741ND). Yet I managed to get one to work again and there was a specific ROM from OpenWRT for that particular device. I tried and it worked beautifully.
However, that might give you the idea that creating such a ROM is easy. But the amount of warnings you get on that website about how easy it is to brick your device... Really, know the hardware in your router very, very well.
The router, is it just for yourself or are other people also depending on the internet connection you plan to use this custom router with? when it is the latter case, you will get a lot of crap from those people the second they think something is wrong with the connection.
On the OpenWRT site I also read the sage advise: If you think you can build something better and cheaper than the devices they build their ROMs for, you are mistaken. First, those devices contain hardware that is proven to work well (enough) together with a decent (enough) degree of reliability and for a price that you will have a very hard time trying to match.
Your custom selection of hardware must prove itself still and might be cheaper if you take a look at the component price alone. Personally, I can't be arsed to go out and try to "beat" a 15 US dollar device that is very easy to replace. and after I found out which model of device I had, the version of the factory ROM and the that it was build for the Brazilian market (yes, that can make a difference), I found the correct OpenWRT ROM, uploaded it to the device, went through the reset procedure as described on the website and now I have a fully functional Linux-based little router.
If you plan to use PC hardware to build your router, I strongly recommend to use OPNSense or pfSense instead. Much more powerful, runs of FreeBSD and you can extend its core functionalities with lots of extras that is readily available or you can create what you need yourself. Believe me, you lust for tinkering will be more than satisfied with either, especially when you like to create intricate network setups.
Don't underestimate the effect custom hardware with insufficient resources can have on your network. OPNSense and pfSense require only very little hardware resources, if you don't deviate that much from their default setup. Still, if you plan to use such a router with a lot of people, you better add more RAM and CPU quickly. A custom router that works fine for your one-person network, is very likely to fail when lots of people start using it. Another thing that people love to overlook.
Idiosynchro-nasties come into play when you build your own custom router, both in software but also with hardware and not all of them are properly documented.
Now, if you still like to play with hardware that is trusted to work and pretty good documented, you might better go for MikroTik hardware. Allows you build your own device (in a similar fashion as building with Lego blocks) and you are allowed to define a lot of your own stuff regarding networking in your equipment with their software. Best of customization with dependable hardware, which can scale up/down to your (professional) needs.
Companies here in Paraguay do use a lot of Mikrotik networking hardware. And for consumer networking gear, TP-Link is very popular (because it is dependable enough, while cheap). You don't know how many miles I had to make between computer shops throughout the whole of Asuncion (capital of Paraguay) and it suburbs, just to find 1(!) store that still had a new, but older model of an 8 port 1GBit/sec LinkSys switch.