Picking up stitches around an armhole or a neckband frightens a lot of novice knitters as many pattern books fail to address this technique at all. For starters, the term is misleading as it sounds as though you simply pick up the edge loops around the piece of knitting, which actually produces a lumpy and inaccurate join. A better term for the process would be pick up and knit, or even knit up, as it better describes what you should actually do. The problems with the edge loop approach are many, it can distort the fabric, leave holes, reduce elasticity and most importantly does not hide the edge. The aim of a neat 'picked up' join is to completely enclose the edge so that all of the minor irregularities which can occur at the edge of the work are hidden. The aim is not the flattest possible but the neatest possible join.
There are a number of different picking up methods available and it is well worth while obtaining a copy of a really comprehensive 'how to' book. There are many on the market but The Knitters Bible by Montse Stanley is a brilliant and clearly explained source for best practise and you can still find copies on the internet. Two of the most common 'right to left' methods are the knitting needle method and the first crochet method (which is particularly useful for left-handed knitters).
Let's start with the knitting needle method, which is quickly and simply demonstrated in the following video for those of you who like to be shown in real time.
'Basically, using a fresh end of yarn you insert your needle in the 'v' of the stitch going beneath the edge which usually resembles a crochet chain (make sure you go beneath both strands of the edge stitch 'chain'). Wrap your yarn around the needle just as you would if knitting an ordinary plain stitch, and pull the yarn loop through from the back to the front. This means your needle now has a loop on it and you can progress to the next stitch.
For a crochet style pick up you can either use your crochet hook (one with a slim central section rather than a fat 'handle') just as you would your needle for the knitting needle method, and as soon as you have a suitably large number of loops on your crochet hook you simply transfer them to your knitting needle repeating the process until you have the required number of stitches. Alternatively, and particularly good for left handed knitters is to have a knitting needle in your right hand, and with your left a crochet hook which you insert in the stitch in exactly the same way as the knitting needle method but hook your yarn through from back to front using the crochet hook then slip it straight onto your needle.
Most patterns will tell you how many stitches to pick up for each section, i.e.pick up 4 stitches for top of raglan, 18 stitches for back neck, 4 stitches for left raglan, 20 stitches for scooped front neck. Others can be less specific, just giving a total number and of course if you are designing your own garments and thus writing your own pattern it helps a lot to know how to estimate how many stitches you need to pick up and knit and how to distribute them.
If the designer has only given a total number of stitches to be picked up they should be distributed evenly along the edge you are picking up from. Given that you will sometimes be picking up directly from a row below, but in other cases along row edges or shaping edges you need to calculate the number of stitches accurately. You can measure along the edge, find the tension of the fabric (swatches very important for your own designs) and then multiiply the overall length in inches or cms by the number of stitches in your tension swatch per inch or centimetre. It sounds more complicated than it is, so here is an example.
To pick up around a neckline once one shoulder seam has been completed, we will say the overall length of the edge to pick up, including all curves, is 20 inches or 50 cms. You tension swatch has say 22 stitches to 10 cms so we multiply 2.2 stitches (1 cm) by 50 cms (length of edge to pick up) to get a total number of stitches (110 stitches). If you end up with digits after the decimal point, I always round down a stitch or even two if picking up for something like a neckline where we are usually dealing with a decreasing number of stitches going into something like a crew or roll neck, or if the picked up section will either stay the same width for a while, or increase I round up to the next whole stitch. Most of the time goes with what looks best!
Another good method is to divide up the whole length of edge you want to pick up with fabric markers or stitch markers at regular intervals (1 inch, 2.5 cms, 5 cms and 10 cms are common choices). Then you just pick up and knit between each marker the number of stitches determined by your pattern or tension swatch, so for markers 5 cms apart and a swatch with 20 sts to 10 cms you pick up 10 stitches between each pair of evenly spaced markers.
Craftsy have an excellent section on pick up and knit and various methodologies for how to calculate stitch numbers and to deal with different types of edge. It is beautifully clear for beginners and can be found here https://www.craftsy.com/knitting/article/picking-up-stitches/ and is excellent for beginners with lovely clear pictures. For those who want to become really advanced knitters I again cannot recommend anything better than the Montse Stanley title in our resource listing as it gives you every technique you will ever need to become able to knit to couture quality, particularly if you want to begin designing your own individual projects.