Percent composition asks what share of a compound's mass one element brings: $\%X = \dfrac{a\,M_X}{M_{\text{compound}}} \times 100\%$. Weigh the X atoms in ONE formula unit, divide by what that whole formula unit weighs, and multiply by a hundred. Notice it needs no sample at all — a speck of water and a lake of water are both 11.1 % hydrogen by mass. It is a property of the formula, not of your beaker.

The trap that eats marks is $a$, the subscript: the number of X atoms per formula unit. In $\mathrm{H_2O}$, hydrogen's contribution is $2 \times 1.0$, not $1.0$. Forget it and every hydrogen percent comes out exactly half right, which is the most expensive kind of wrong. Two free checks: an element's share can never exceed 100 %, and all the elements' shares must total 100 %. If your oxygen and your hydrogen add to 94 %, something upstream is lying to you.