The Squares with a circle inside them must contain the numbers 1 to 3, so on puzzle 65, R5C5 is a square with a circle, so must contain 1, 2, or 3. It cant be 1 as there is a 1 in R1C5, and it can't be a 3, so it is a 2.
Similarly the Squares with Squares in them must contain numbers 4, 5, and 6.
Also blank squares contain the numbers 7, 8, and 9 --> look at R8C4
For 1, 2, 3 just look at Squares with circles
For 4, 5, 6 just look at Squares with squares
For 7, 8, 9 just look at blank/normal Squares.
If you need any more help, don't be afraid to ask