If you can change the direction of your flash, then bouncing is the cheapest way. You can direct the light to the side or up, and then hold a large white paper, and bounce back the light to the subject.
Or with available light, you can use north facing window or you could wait for cloudy days to get diffused light.
But I think getting the flash off camera is the 1st thing to improve photography as Rosie suggested. I would go with cheap radio trippers instead of wired remote flash. Then you can use soft boxes, or umbrella (either reflective or shoot-through).
https://www.fasttech.com/products/0/...tudio-umbrella
https://www.fasttech.com/products/0/...tudio-umbrella
For me, umbrella is easier than softboxes to set up.
Here are a couple DIY ideas:
How To Build 24 DIY Softboxes - DIY Photography
Before I got the lighting setup (it improves photography much more than buying a new camera), I also used something like what Ray mentioned. I used a cardboard box, put the aluminum foil inside, and covered one end with thin fabric, and made a hole in the back to stick a flash head into.
If you don't want to go with remote tripper route, you can use high power CFLs in the softbox.