Start gently. If you try to go for big notes early you'll overtax your vocal muscles.
Step2
Make a yawn sound quietly at the top of your range (falsetto or head voice) and slowly descend all the way to the bottom your range. Repeat 5 to 7 times.
Step3
Start in the middle of your range. Sing a series of five notes up and down (the first five notes of a major scale) using a gentle vowel like "ah". Repeat half a step higher and so on until you start to stretch. Stop before you get too high.
Step4
Do the same thing going down a half step repeatedly until you reach the bottom of your range. (It's much harder to blow your voice out singing too low.)
Step5
To work on flexibility, sing a note near the bottom of your range and then the second note in the major scale. Return to the first not. Then the third note in the major scale, Then return to the first note. Do this until you jump an octave. Repeat gradually getting higher using each of the vowel sounds.
Step6
Sing a song gently. The national anthem is a good song that covers a wide range.
Step7
Sing. Don't warm up any longer. Your voice should be warm and well ready to perform.