I just use a needle and thread like you were going to put a button on a shirt. Hand tack but I have experimnted lately with just sticking it under the machine needle. But I really don't thiink it works as well. It really doesn't matter whatever holds the tack in place works for me. When I get down to the end of the thread I will pull the thread so I have a single thread and use up the end that way.
I also thread about 10 needles at a time and have them on the side of my pincushion so they are handy and I don''t have to stop the flow when I get in the groove and am really getting stuff done. Very annoying to have to let go of what I am working on and rethread needles.
My tack is just a couple of stitches over each other in the same place and I leave a small 1/4"-1/2" tail so I can see it is there.
Word of warning my tacks aren't always an instant answer. You will still have times when it moves and your seams aren''t where they are supposed to be but the tacking is better than pinning in my opinion.
Check to see whether your seams meet where they should before you sew and hopefully you will have less redos
Here's a sample of my tacks. Sorry it is not half squares but you get the idea.