How can I fix a stair to its shape?

Short answer: it can't be done. With certain mods however, it can possibly be imitated.

The way you are facing determines the direction the stair faces.

If you place two stairs facing different directions (eg, one facing north, one facing east or west) they will form a corner.

When you destroy a block, it will revert to it's original shape.

It also depends on what version of Minecraft you are playing, because corner stairs were released in version 1.4.2. Any version before this will not have this function

I know most people are going to think that I'm just necroposting on a 6 year old post, but I think plenty of people end up seeing this post and I happened to find an answer!

First, I want to note that this can only be done in Creative, but you can switch your gamemode back to survival and it will still be there.

The first step to doing this is by creating a new Creative world or switching your gamemode to Creative via cheats. The cheat to do this is /gamemode creative and simply type this into the Minecraft text chat. Once you're in Creative you will need to give yourself the Debug Stick by typing the cheat /give @a minecraft:debug_stick. Now you're ready to change the shape of a stair!

Place down your stair wherever you want it to be as if you break it or place a block next to it you will have to repeat the process. Now hold out the Debug Stick and left click (M1 on PC) until it says "selected "shape" (straight)"

After that simply left click until your desired shape is chosen.

You can now put your stuff away and the stair will stay in that shape as long as you don't break it or place a block next to it.