For analog video, peak white or single R,G or B component should be +1.0V, black +0.3V, and if it has sync (sync on green or composite video) 0V for the sync pulses.
Usually, TTL RGB => SCART RGB was a simple series resistor to form an attenuator with the 75 ohm SCART RGB input impedance, The resistor was typically somewhere between 180 to 330 ohms depending on how high the TTL RGB output swung under load.
If the black level overdrive (dropping to 0V rather than 0.3V) is a problem, or if you need to add in an intensity signal, it gets more complex and you'll probably have to build a proper level converter.