Right means, of course, fitting for the dance. Different groups will differ somewhat in their likes and needs, so everything here should be considered only approximate. You are trying to satisfy your dancers, not me. So feel free to modify anything here to fit your dancers.
The best general advice is to find some musicians that your dancers like, and insinuate yourself into their circle as an apprentice. If your dancers use recordings much, get some of them and listen to them. But mostly, try to find a bunch of friendly dancers, volunteer to play for them, and ask them for advice. They will probably have problems explaining things in terms that you understand, but try to not get too frustrated by this, and with time it'll work out.
In any case, it is important that you be able to hold a tempo. This is often difficult for musicians who haven't played for dancing, and it is useful to practice with a metronome until you find that you no longer have problems staying with it.
SCD is closely related to contra dance, so if you've played for contras, you know much of what you need to know. But there is one interesting problem that contra-dance musicians often have with SCD: The tempo is noticeably slower. This is due to the fancier footwork and the more open nature of the sets, with more ground to cover in each measure. Very often, contra-dance musicians are accustomed to their faster tempo, and unconsciously speed up to their "normal" tempo. This makes for very hectic SCD. Work on the slower tempo.
A technique that SCD musicians use (often without being aware of this) is to play in a more "dotted" fashion. Reels can sound somewhat like fast hornpipes. Jigs normally have the first of each 3-note group dotted. This sort of not-quite-even rhythm makes for more interesting music at the slower tempo, and will help you keep the tempo steady.
You will also have to learn to play strathspeys. The standard tempo for these is around 60 beats per minute, more or less. The same remarks apply here. The expert dancers will really like it if you can play more slowly, but beginners will find this very tiring.
Most of the SCD recordings have piano and snare drum backup. Either of these is very helpful, especially if the pianist or drummer is a Scottish dancer, knows the tempos, and is willing to act as a metronome. There is a common problem of pianists that want to do things that are too complicated. It can be frustrating to be playing an instrument like a piano and using it only for a simple bass-chord backup. But this is what's best for the dance, so if you have a pianist who is happy playing chords, treat them very nicely.
Nowadays, most of the dancers have learned to dance to the commercial recordings, which usually have complex medleys of tunes that make them more interesting to the listening audience. As a result, dancers tend to expect such medleys. And since it's fun in its own way, why not?
The typical Scottish dance will go through eight times, with each of the four couples active twice. Some dances are for fixed-size sets, and require a particular number of "rounds" of the tunes, usualy 4, 6 or 8, but sometimes other numbers.
Also, at least half the SCD repertoire have what are called "title" or recommended tunes. In some cases, the dance and tune were written together, for some special event. In other cases, the dance deviser thought that one tune particularly fit the dance. For dances with a title tune, the usual practice is to start and end the medley with that tune, and find other tunes with a similar feel for the rest of the medley. The alternate tunes will usually be in different keys.
There are several common order of the tunes. If you use three tunes, most of the recordings will use the order 1-2-3-1-2-3-2-1 or 1-1-2-2-3-3-1-1 to get 8 rounds. Sometimes you will hear medleys of four tunes, usually 1-2-3-4-2-3-4-1. Occasionally, tunes will be used that are half or twice as long as the dance, and then you need to be inventive in the ordering. For half-length tunes (which includes most strathspey tunes), it's normal to play each tune twice for each round of the dance.
As an example where you need to be inventive, consider the Duke of Atholl's Reel. The standard tune for this is the Atholl Highlanders which is a 64-bar jig-time march. The tune is twice as long as the dance. If you were to medley it with two normal 32-bar jig-time marches, you'd want an order like 1-2-2-3-3-1 or 1-2-3-2-3-1, which would give you 8 32-bar "rounds".
You should be prepared to play your medleys in different arrangements at different times. Thus, if the number of couples isn't right for 4-couple sets, you may need to do 6-times or 10-times medleys. Any order of the tunes will work, with the qualification that if there's a title tune, you should make it the first and last tune.
Such dances almost always have a special tune that fits the dance's phrasing. This is also an opportunity for some creativity. You can get a medley for such dances by listening to a recording and copying it. But far more interesting is to learn the dance's title tune, and start thinking of other tunes that can be modified to match it.
For dances with 5 or 6 8-bar phrases, this is usually not difficult. Find tunes with the same feel, and try arranging their parts to match the dance. For a 5-phrase tune, instead of the usual AABB form of most tunes, try AABBB, ABABB, and AABBA, with appropriate arranging of first and second endings. If none is very satisfying, it's not a good tune for the dance, so go on to another. Or write a new phrase in the style of the tune. (Tell people that your grandfather played it that way.)
For some examples of this, the 40-bar dance Mairi's Wedding is very well known, and there are a lot of good recordings. The title tune is actually a song, and when sung it is usually just 16 bars, two 8-bar phrases. The recordings play it ABABB or AABBB, or AABBC, where C is a made-up variant of the B phrase. The other tunes in the medley will often have their parts ordered differently, however fits each tune.
One of the trickier aspects of playing for SCD is getting these 40- and 48-bar dances phrased right. People can play "on autopilot", and do a tune AABB like they have always played it. A good SCD musician won't fall into this trap (often), but will be constantly aware of the dance phrasing and how the tunes are being modified to fit the dance.
The best way to learn this is to simply do it. Pick some tunes that you know well, decide on an unusual order of the parts, and play them that way until you don't stumble.
... More to be added as I think of them ...