What troubles are you having? Strips too long? Puckers in the seam?
How are you joining those strips that make the pane? With miter corners make sure you have a 45 degree corner. You can also use a square sashing block in the corner to make the connections between the two strips.
Are the window blocks cut to the right measurements and squared up? They may be rectangle in shape but should be straight on the edges.
What seam allowance are you using? Check the strips to make sure they are at the right size for attaching to the block.
You may have to mark the seam line in the corner. Mark 1/4 inch on both pieces with an intersecting line into the (inside) corner. Stitching down that line, stop right on that the corner mark, pivot the block and continue sewing the other strip.
Hope that helps & Welcome to the boards!