For my vintage Singer, I bought a 1/4" foot with a built-in edge guide. On my Bernina, I have a 1/4" foot (no guide), but also bought an edge guide that screws into the machine bed. I like that much better. It has a longer guide & I can also adjust the width for when I'm sewing borders or binding, which I do with a 1/2" seam. It was the best $20 I ever spent.

I would recommend a 1/4" piecing foot. Also, I keep a small square ruler, maybe 3 or 3 1/2" with me and check as I sew. You could also try simple paper piecing.