I've been asked how do I do that, those pretty sweaters.
There's no easy answer since there are many knitting techniques to start with and an endless choice of possibilities in using them but I'll concentrate on what I'm doing these days: a garment that is simply constructed and made of several different yarns that are selected for a desired colour and texture effect.
Captain Obvious' comment: First, you should know your measures. Not your clothing size. Tables do exist but it's always better to measure the wearer. Good trick is to take a fitting garment similar to the desired result and measure that one.
Now, I've been known for collecting yarns. I get them from various sources in quantities that are available so I'm bound to improvise. It is quite easy to have a burgundy red sweater in your mind and go into a yarn store and pick a burgundy yarn in solid colour that just convenes to one's taste in texture and hand. Now, having one skein of this and one skein of that, where this may be cotton chainette in swamp brown, 40 metres per 100g, and that being laceweight merino in shades of lilac, 400m per 100g, is more of a challenge. These yarns wouldn't go together to make a wearable garment. Maybe a lace shawl made of that merino, where cotton chainette would be used as piping around the edge of the shawl. I'm not sure whether it would work but it could. I'm not trying it, should you want to know.
A practical example: I got ten skeins of Noro's Suzuran which has been discontinued for long. It's thin and thick yarn and it's printed. The resulting fabric is somewhat uneven, with nice stitch definition and small blobs of colour:
It doesn't look entirely bad as such but...
In my knitting, I'm a purist. I knit. I don't sew, tweak things around, I knit. I prefer to have any garment knit in one piece. Admittedly, one reason is that I cannot do any seaming really neatly but I like the engineering process: I start at one end and finish at another and then I'm done with that. The advantage of knitting is that one can shape the fabric on one go so why not use this possibility to its best. I love to experiment with various ways how to construct a garment and one reason why I like striping yarns is that the process can be seen.
... the resulting fabric is blah. The colours match nicely, the texture is interesting but they clash each other and at the end, it's not possible to discern where it begins and where it ends. The sweater would be one of those bland garments that do not actually clash with anything but neither they match anything, themselves included. Just blah. I decided to combine this yarn with something else and after some digging in my Stash of Doom And Two Days More, I found a few yarns that might work colour-wise:
Noro's Shirakaba, silk/cotton/wool blend. The colour is outstandingly beautiful.
Noro's Akogare, colourway 19 (1), another pretty yarn. This one is striping and the colours of Akogare and Suzuran go together well. So... let's give it a try:
And we get clown puke.
Shirakaba is much shinier and stands out too much to create a pinstripe effect which I don't want. Akogare is too colourful and it's not the thing.
The last attempt for now:
Leftovers of Tsubaki. I've used this yarn for another sweater but I kept cutting out the colours that didn't match my chosen colour scheme and these are the bits and pieces of the other colours. The mustard yellow contrasts nicely both blues and greens, greens and blues and the cold purples are at the same end of the colour spectrum.
Speaking of matching colours, beauty is in the eye of the beholder. I do have my preferences and I tend not to have things too wild. In the last yarn-matching picture, there's yarn in blue, cerulean, magenta and neutral earthy brown. And another yarn that contains green, cerulean, cold shade of pink, cold shade of lilac and mustard. The only really contrasting colour is mustard, the rest play together nicely. Tsubaki is a striping, or rather colour-shifting yarn (there are no breaks, green slowly changes into pink etc.) so mixing it in will show the construction of the sweater as it can be partly seen here:
(It's somewhat hard to explain the construction without sketches but I may write down the pattern someday, should someone want.)
I'll keep you informed about the progress, want it or not.
(1) This yarn is discontinued for long but they still have some here. NAYY.