You don't. Don't be silly. A tenth of a teaspoon is a pinch (a pinch of salt, etc), or a few drops of fluid.
I'd be suspicious of any recipe giving such instructions.
Very rarely, recipes require very accurate measurements, for example in the molecular cuisine. Those should give the required quantities in volume or weight (ml, mg, etc). Those that do not should stick with more common descriptions such as "a pinch" or "a few drops."
One eighth is written " 0.125 ".The word or unit after it tells what it's one-eighth of .
That IS the decimal form.
It is in decimal form!
That IS the decimal form.
That is in decimal form.
You write fourteen and three tenths in decimal form as: 14.3
0.09 IS in decimal form.
It is already in decimal form.
It is already in decimal form.
7.32 It is in decimal form.
137 is in decimal form.
2.4 is in decimal form.