First off, here is a
LINK for a free web-based Photoshop, in case you don't have one already. It's not what I use, but I have used it in the past, and it works quite well.
The easiest way I found to accomplish it perfectly still takes a bit of work. I start with placing diamond gemspark block behind what I'm going to make transparent. It serves two purposes. One, to light the blocks so there aren't awkward shadows, and two, so it's easier to Photoshop.
View attachment 88168
Then, I put on familiar vanity sets, and use an invisibility potion so I don't get in the way for the screenshot.
After that, I take the screenshot to Photoshop. I find it easiest to use an eraser tool to completely white out the edges around the block. Will look like this when I'm done erasing (this takes quite a bit of time):
View attachment 88169
Next, I unlock the background layer (double click the lock icon that I circled):
View attachment 88165
Then use the magic wand (also circled), and click on a white area. You'll know if it's selected, because it will look like ants are crawling along the edges of it. Once it's selected, hit [delete]. Do this for all the white areas that you don't want. Once an area is deleted, there will be a gray and white checker pattern in the background. All that means is that the background is transparent, so don't worry. It will not be visible when you view the image outside of Photoshop.
View attachment 88166
Crop the image to the size you want, then save, and you're done ^.^
View attachment 88170
There may be an easier method to doing this, but this is the way I taught myself to do it

Also, note that if you want to do this to something that is white, or has a white outline, you'll want to use a gray or black when doing the erasing bits, otherwise the magic wand will auto select areas you don't want.