Knowing how to work neatly into row ends is a very useful skill in crochet. By doing so you can reduce the need to seam and work seamless and modular designs. It can be used on the turn of a heel or to add a button band or collar to a sweater. And, as crochet ribbing is almost always worked sideways, it allows you to work hats brim up and socks cuff down too.
There aren't any hard and fast rules about how and where to place you hook when crocheting into the row ends. And, other than where you are placing your hook, the stitches are made in exactly the same way. So its not really too tricky to master.
Recognising what a row end looks like is the easiest way to start. Obviously each stitch will look a little different so here are some marked examples of the different stitches with each row end marked with an arrow.
I find the shorter stitches harder to identify the row ends but conversely much easier to actually work into.
Here's what a piece looks like when picked up into the row ends.
The above is worked into rows of double crochet in the back loops only at a rate of one stitch (a double crochet/US single crochet) per row end.
There's no right or wrong place to put your hook as you work but you will get a neater effect if you choose the same place every time, so look carefully at the stitch as you place it and think about which strands you are going under or over. And remember, the more strands you go over the more firm the join will be, if you want a looser more flexible join go closer to the edge.
Often in my patterns I create a rib of half treble in the back loops as this looks like a knitted rib. When I instruct you to pick up stitches I'll often tell you to put the stitch or stitches into the row end that looks like a knit stitch or a "V". The photo below shows where these are.
The number of stitches you pick up per row end will probably be set in the pattern but if it isn't and you want to work so that the fabric lays flat you can use this rough reckoner - put the same number of stitches into the row end as the number in the turning chain..