The first suggestion is to use hands on activities to teach math. You do not need to break the bank purchasing commercial manipulatives though there are some I like very much. You can do quite a bit for students with $8 worth of craft sticks and some hairbands from the dollar store.
Begin by teaching students numeracy patterns. I like to build number quantities with dice because they are everywhere in our society. Use dice patterns to six and then work on combining two dice to make a patterns to nine and ten. Do not use more than three dice. The object is to recognize the quantity without counting. Link the patterns to numbers and number sentences. Throw two dice in a bowl and see if you can name the combined quantity. Then, apply those facts across place value. If 2+3=5, then 200 + 300 = 500.
Be careful of the numbers between 10 and 20 and all tens to one hundred. The language changes and for some students that is a problem. You must explicitly teach the numbers between 10 and 20 because they are Anglo-Saxon and "backwards" for language. The "teen" means ten. In most numbers above 10 we say the tens and then the ones. Think of 23 as two tens and three ones. BUT, those silly Anglo-Saxon numbers...well, they say the ones and then the ten. No wonder our little ones get confused.