An easy way for you to join the squares together and make sure they are all in line and squared - up:
Measure and cut your vertical sashes (the strips that go between the blocks when you place them in a row, right to left). Make sure they are the length your block is supposed to be-not the actual length of your block.Then pin the top of the sash to the top of your block,right sides together. Find the bottom of your block and pin the bottom of the sash to it, right sides together. If the sash lies flatly against your block it is not a problem, sew the side sashes onto your blocks and join them into horizontal rows.
Then get a long piece of sashing and cut it to the length of all your blocks in each row plus the width of all the side sashes in each row.Mark the sash points and block point measurements along the entire sash (making sure you calculate the seam allowances being taken out or still in).
Then pin each sash and block to its correct mark and sew.
If at any time you find the sashing does not sit flatly, you will have to either measure and redo your block, or square it up before you proceed. If it is only a little bit out, you can ease the extra measurement in as you go but DO NOT change the length of your sashes or sash marks or your quilt will not be square.When you have the centre of your quilt sashed together, measure down the centre of it in both directions to determine the correct lengths to cut the outside sashes and use the same method to attach - you may need a few more pins to find if you are accurate.
Hope this is clear and not too long winded. There are many short cuts after you become more proficient. Happy quilting.