I bought a ROISCOK RP-208CN. Can I connect two sensors in series to one zone? Also do I really need to connect the tamper cables? The whole system will be within the home, there are no external cables or sensors.
I’m not familiar with your particular alarm system, but there are two types of sensors, normally open and normally closed. Normally open sensors are like a switch that is off. When the sensor is triggered, it’s like turning on a switch and current flows.
Normally closes are the opposite. They are like a switch that is always on, until the sensor is triggered, and then it turns off. The advantage of normally closed is that there is always current going through them, so if someone breaks the wire, the alarm goes off. If someone breaks a normally open wire, the alarm system won’t detect it unless you also have a tamper circuit which has current flowing though it.
If you have multiple normally open sensors in the same zone, you wire them all in parallel so that if any sensor is triggered it closes the circuit and sets off the alarm.
If you have multiple normally closed sensors, you wire them all in series so that if any of them breaks the circuit, the alarm goes off.
If you don’t connect the tamper circuits, you may not know if someone cuts a wire or if a wire just breaks. Depending on your particular alarm system, you may have to short the tamper contacts if you don’t have the tamper wiring in place, otherwise the alarm system will trigger the alarm.
Thanks mate, that’s exactly the info I was looking for. 
And another question. What is the best way to connect the cables? Simply twisting them together and applying some electricians tape will do? I want the job to be as clean as possible so another technician won’t have problem finding their way.
Wire nuts are always preferable over twist and tape splices. They hold the wires together, plus providing the needed insulation.
When you wire the circuits into the terminal strips on the panel, if the wire comes into the enclosure from above you’ll want to make a U-shaped loop in the wire so that it comes up to the terminal from below. This keeps water from following the wire right onto the control panel.
Also make certain that any circuits that need an end-of-line resistor have them, or else you’ll have trouble when you fire up. Usually tamper and fire alarm circuits are supervised (that’s the term for a circuit using a EOL resistor).
Mark and screw in the transformer that will power the system. Mark the circuit breaker that the transformer is plugged into at the breaker panel.
Make sure that whatever speaker you use as the notification device is the correct impedance for the on-board siren driver, or you’ll blow the driver.
If the panel uses on-board fuses, it doesn’t hurt to have a spare in case something wasn’t wired right when you fire up the panel.