the first thing to understand is that they have to grasp the idea/concept of addtion before you can write anything on a page.
So it is all about practical maths.
Have a box of toy cars. Get them to pick up 4 cars, count to check they have 4. (make sure they can count correctly, not count 4 cars as 5 cars,) Then hand then 2 more cars and ask how many have you got now?
All the time using maths sentences
you have 4 cars if I give you 2 MORE how many do you have now?
If you had 3 more cars how many would you have all together? etc etc
Do the same with subtraction. - how many would you have left if you give me 3 cars?
Do it all the time with everything. - There are 2 plates on the table we need one more - how many now?
I have 3 spoons and there are 5 in the drawer, how many all together.
If you have 3 sweets and I give you 4 more, how many sweets would you have etc etc
Start with number up to 5, then 10. Don't go beyond 10 until they are really confident doing it up to 10.
Once they are confident to count add numbers to 10, then you can add the written numbers.
Make number cards 1-10
match written numbers with the correct quantity ( 3 pieces of lego on the 3) then add the + sign and do simple sums, but alwyas have concrete items there too, so 3+2 = but you have 3 pieces of lego and 2 pieces of lego. Then move the pieces over to the = and count how many all together and add the number card.