A quarter inch foot may have come with your sewing machine. If not, you may be able to buy one for your machine or buy a generic one. It is well worth the expense. If you sew with the fabric just at the edge of the foot, and not beyond, maybe even a few threads closer to the needle than the edge of the foot, you should have a quarter inch seam.
Put a ruler under your needle and mark with masking tape where the quarter inch mark should be. If you use several layers of masking tape, it makes a little lip to work against. Electrical tape works well too. You can buy a magnet that will serve this purpose.
CAUTION: Do not use a magnet if you’re using a computerized sewing machine. Magnets ruin computers!
Practice making an accurate 1/4″ seam using scrap fabric until you are very good at it. Cut two pieces of fabric 2 1/4″ wide; sew them together and iron, checking afterwards to see if they are 4″ wide.
After washing your fabric use spray starch while ironing to replace the sizing. This makes it easier to get that accurate seam.
Check each block after sewing to make sure it is right size, and make corrections before sewing them all together.