Music theory question: how do you use cadences with modes?

Modal cadences are - necessarily - weaker than tonal ones (ie in major or harmonic minor) - but that should be seen as a positive. The whole attraction of modal music (at least for musicians in jazz or rock) is that it avoids the "perfect" cadences of key-based music.

Naturally that means you need other ways of establishing a tonal centre (assuming you want a clear tonal centre...). The common way - in both jazz and rock - is simply to just play the root chord constantly, or a lot more often than the other chords. You create an "aural groove", as it were, just by repetition.

In "modal jazz", the whole point of it in the first place was to escape tonal harmony. Musicians like Miles Davis (in fact primarily Miles Davis) were fed up with the frantic roller-coaster of functional bebop progressions, and wanted something "cooler": more "static" harmonically, and freer melodically. In order to avoid any hint that their chords were functional, they employed quartal harmony rather than tertian. I.e. for D dorian mode they wouldn't use Dm7, because it came with functional baggage: it sounded like a ii in C major, or a iv in A minor. So they'd build a kind of Dm11, maybe voiced D-G-C-F (with or without an A on top) - a chord that (usefully) gives no hint as to where it might be going, if anywhere.
Modal jazz chords, then, tend to sound like unresolved suspensions - which was the whole point. Nothing resolved (harmonically at least). Chords just hung there, in open-ended ambiguity. Cadences of any kind (even modal ones) were avoided. This is typical. (What they did retain from older jazz standards, mostly, was the 32-bar AABA form.)

In rock (to generalize), it's different. Few rock musicians know or care about modes (although some seem to think they do), but they know they don't much like those classical V-I cadences (too neat, too pat), and do like the flattened 7ths of blues, and of some folk musics, and the drone effects of ethnic music such as Indian or North African. (Raga is archetypal modal music, 100% pure.)
So they start with standard, off-the-peg, key-based sequences - because that's what they grew up with, copying popular stuff, learning the basic tertian guitar chords - but add (what we would call) modal inflections. In a major key, they'll typically avoid V-I, much preferring IV-I, and will borrow chords from the parallel minor: bVII (almost always), bVI, bIII, minor iv. They kind of blur the boundaries between parallel major, mixolydian, dorian and aeolian: a practice known as "mode mixture". They feel no need (unless they've studied some beginner theory and foolishly think it's "rules") to stick to one diatonic major or minor scale.

Cadences? I'm very familiar with modes but what do you mean by cadence?