Do you have experience with working with mains voltage? If you don’t - then STOP. You need to learn more about what you will be working with, because mains voltage is anything -but- forgiving (true, it isn’t as dangerous as say 440V 3-phase - but we ain’t talking industrial automation, either).
Regardless of whether you have the experience or not, and regardless of whether you use regular relays or solid-state relays - make sure you keep the low-voltage and mains-voltage sections of your wiring (or PCB traces) as separated and well-marked (and color-coded!) as possible. Use standard color coding for the mains-voltage section (look up the colors for the country you are in - it varies) for neutral/common, hot and ground/earth. For the low-voltage (control) side of things, use a different color code (there isn’t a real standard - you could apply the PC color coding of black=ground, red=+5V, yellow=+12 volts, and green/other colors=signal or other uses) - and stick with it for everything.
Believe me - it is no fun hooking things up only to find yourself accidentally grabbing the mains wire that is energized (though why it should be is another issue - never work on a circuit with -any- voltage running through it!) - or plugging in a wire and having mains voltage blowing up (and catching on fire) your Arduino.
Also - are you familiar with the “one-handed” rule when it comes to troubleshooting a circuit? If not, familiarize yourself with it. See this article, under “Safety Basics”:
Lastly - with SSRs, if you want them to handle their rated current, they -will- need to be mounted on a heatsink, if they are designed to be - and generally, any of the SSRs that can handle more than a few amps are designed to be mounted to a heatsink…just something to keep in mind. Electromechanical relays don’t need heatsinks.