I heartily recommend first to concentrate on the concepts and strategies, and only later add in random drills. That is true, whether it is addition, subtraction, multiplication, or division.
I've made a video about strategies for learning addition facts. In it, I list several strategies to learn addition facts for first and second grade math. I show the pattern of "Sums with 7", which also is used with other sums, then the 9-trick, the 8-trick, the doubles, doubles plus one more, and how to do random drill using the structure of the addition table.
Then, a new video of mine about strategies for subtraction facts. I recommend the usage of FACT FAMILIES in order to learn the basic subtraction facts. That way, when children have a subtraction problem, such as 7 − 5 = ____, they will learn to think through addition and fact families: 5 and 2 and 7 form a fact family, OR that 5 + 2 = 7, so 7 − 5 = 2. The video also shows a fun tool for subtraction facts called number rainbows!
Hope these are of help!