Take, for example, a decimal is 2.5. Say this number aloud and you will realize that the fractional part is the "five-tenths". Write down the fractional expression after the comma in a simple fraction - "5/10". In the end, the initial number will look like this - "2 5/10".
Reduce the fractional part of the resulting number. For this purpose, the numerator and denominator of a fraction is divided by the same divisor. In this case the number "5". So, "5/10" is converted to "1/2".

So the original fraction will look like this - "2 1/2".
In order to make the number complete, you can submit it to the integer portion also in the form of a simple fraction with the denominator "2". So "2" is "4/2" (divide the numerator and denominator at each other, and you will receive "2"). Now to the "4/2" add "1/2" to get "5/2".

Thus, the original decimal of "2.5" has become a common fraction "5/2".

Put any other decimal fraction into an ordinary fraction by analogy.