Try using concrete examples of the subtraction concept with her. For example, make up real life scenarios. You'll need to do this many, many times using various quantities and problem scenarios to help her grasp and understand the concepts involved.
e.g., (for a family of 5)
1. Here are 5 cookies on a plate for the people in our family.
2. Count the cookies on the plate together and have your daughter write down the number 5 on her paper.
3. Daddy (or mommy or whomever) said they did not want a cookie.
4. Have your daughter remove 1 cookie and put it back in the cookie bag.
5. Explain that the minus sign and the number 1 must now be written down after the number 5 to show that one cookie was taken away.
6. Have your child count how many cookies are left on the plate and show her where to record the number 4 after the = sign.
When students struggle with the subtraction concept, you need to apply it to real life situations, remembering to record and write the number sentence (equation). In time, it will begin to make sense to them.
As they learn the concept with smaller numbers ('lesser' numbers in math terms) you can begin to use larger numbers ('greater' numbers in math terms).
Don't forget to use some addition problem scenarios in there once she has the subtraction concept down. In first grade, they need to become aware of when to use the appropriate operation (addition/subtraction) to use when solving math problems. Knowing which operation to use to solve a problem is a key indicator of the child's understanding of the concepts.
Ooops! I forgot to tell you that she should be asked to draw pictures to accompany each step of the problem. Where I live and teach, every problem that a student solves must be accompanied by pictures/drawings to assist in the proof of their answer. It also helps to clarify the thinking of the student.
Draw a big circle (plate) and draw 5 small circles on it to represent the cookies.
When 1 cookie is put away in the bag, draw an X on 1 of the cookies on the plate to show that it was taken away.