If you pee in a cup, you dip one end of the paper in the pee. Or you can press one end of the paper to your tongue to test saliva. If you are bleeding, you touch one end of the paper to the blood.
The litmus paper will change color based on your pH. You compare the paper to the little chart that's likely included in the pack of papers to gauge your pH. Or you can use the rainbow colored chart in our first image of this post - blues and purples are high pH = more alkaline. Yellow/red/orange are low pH = more acidic. Greens are around neutral = 7.
There are also electronic pH meters that give an automatic reading when you use them in liquid.