Making an illuminated manuscript takes time and effort, though can be a real boost to a conworld's artistic culture.
One could do it the traditional way. First, create a sketch, then add gold leaf (could be golden foil or just gold ink) before adding other colours, like reds and blues, to make the picture. The writing is normally beautiful and often uses calligraphy. Due to the time taken to illuminate, most manuscripts limit illumination mainly to pictures and first letters, though your world could be different.
However, if one wants to save time, one could just get out some coloured pens or pencils and make the colours like that.
One thing to consider when using illumination in your conworld is where it would be used; most of the time, it would only be used for important or religious documents. They can be used to show many things, like wealth, power or holiness; decide what illumination will convey.
Sometimes, single letters, normally capitals, are illuminated and have a story of figure hidden in them. You may want to consider making your world like this, to save time.