First, you probably all know how to do a screenshot - I didn't - probably one of the last few in the Western World! But just in case, for most pc's you will find a button labelled "Print Screen" just above the "Insert" key in the block of buttons to the right of the main keyboard. On my laptop it is the row of buttons at the top just to the right of F12 and is labelled "prt sc". On some laptops you might need to push the function or shift button at the same time to get to it.
Now, it doesn't actually print the screen - it saves it to your clipboard. Then you need to crop it to just show the bit you want to copy in to your blog.
There are probably heaps of different ways you can do this but I did this (sorry Mac users I don't know if you can do it like this):
Open Paint. You will find this by clicking the Windows icon right at the bottom right of your screen. Then click All Programs, then scroll down to Accessories and you will find Paint listed there. I actually dragged it onto my bottom toolbar so I can get to it quickly each time I want to use it.
Then click Paste - at the top left and bingo, your screenshot will appear. Then you have to select the area you want to copy. I found the best way to do this was to click select (just a bit to the right from paste) and then click free form selection and then go back and click rectangular selection - for some reason this makes selecting easier. You then click at the top right of the part you want to copy and drag it diagonally until you have it all in the box. Then click Crop (which is one to the right of Select).
Then all you do is go to the Blue tab above paste and click on the little arrow beside it and it will give you an option to Save As - save it in whatever folder you want (I now have one called Blog Tips). You can then also edit the image to include an arrow or circle etc if you like.
Then you just insert it into your post the way you would any image.