One idea runs through this large family: order, things put in their proper sequence. The bare noun סֵדֶר is order itself, and it is the name of the Passover Seder, the meal run in a fixed sequence. To arrange is לְסַדֵּר; the arrangement, סִדּוּר, is also the prayer book, the fixed order of prayers. From the same root come the typesetter who orders letters, the regular (סָדִיר) thing that recurs in order, the series (סִדְרָה), the corridor (מִסְדְּרוֹן) that lines rooms up in a row, and even the labour union (הִסְתַּדְּרוּת), people organising themselves. One root, and the whole idea of putting things in their place. Every word here is verified against pealim, niqqud included.
The same root runs through the verb system. Each binyan is a voice: it bends the one idea into active, passive, causative or reciprocal. Tap to hear.
The everyday verb is the pi'el לְסַדֵּר (to arrange, tidy). The hif'il לְהַסְדִּיר adds a more formal sense of regulating or settling a matter. The reflexive hitpa'el לְהִסְתַּדֵּר (to manage, to get organised, where the ת becomes ת after ס with metathesis) gives the union noun below.
Same root letters, different noun moulds (mishkalim). The mould tells you the function: who does it, the act of doing it, or the thing produced.
Strip the prefixes, suffixes and vowels. Three consonants remain, and the meaning lives there.