This is how you count your stitches. The example in the picture is for 18 stitches.
At the beginning of your rounds, this is the "top" view. This will be the "OUTSIDE" of your amigurumi.
And this is the bottom/back view, which is where the tail is and this will the be "INSIDE" part of your amigurumi.
The picture below is how it looks like when you've turned your work "inside in".
Next are the instructions on how to place your stitch marker. There is no one way of doing it though, this is just how I do it, and maybe it'll be helpful to you.
Firstly, have a piece of different coloured yarn, place it like the next two pictures.
Insert your hook into BOTH loops.
Yarn over and pull through. Done! The stitch marker will stick out like the pictures at the top of this post and you simply continue crocheting the round.
Note: I know I skipped the yarn over and pull through pictures, but it is hard taking pictures with one hand and holding the yarn with the other hand at the same time you know...