Things to keep in mind:
You are doing this to have FUN
The improvisational method has two important “rules”
If it's too long, cut it off.
If it's too short, add to it.
Cut several strips of varying widths from each fabric you intend to use for the quilt. I find that if I don't cut the strips beforehand I tend to never get around to using that particular fabric. I'll get into cutting the strips a bit further on.
While working with small pieces, it's no big deal to cut with scissors. But once the units get a bit bigger I do recommend switching to a rotary cutter and ruler.
I attach pieces and pieced sections of fabric to strips (and then whack off the extra length of the strip) because it is really easy. But you can always use a scrap instead.
During the early phases of construction, there is no proper size. Nothing has to be just right. Add or subtract, it doesn't matter.
A consistent 1/4" seam allowance is completely unneccessary; however do try for a straight seam. Do keep in mind the use of the quilt - if it's going on the wall then a narrow seam won't be a big deal. If it's going to be a lap, bed, or crib quilt then closer to 1/4" the better. You can always cut down a seam allowance that is bigger than a 1/4" if you are going to be hand quilting, otherwise who cares? Well, I don't care anyway. Maybe people who actually machine quilt might have an opinion on that.
It isn't necessary to use an iron while working on the units (that's what I call the letters and numbers before they are finished to their final size, when I then start calling them blocks). If you want to, knock yourself out. Once you are trying to make things a certain size, then it matters and pressing with an iron is important.
You're making folk art. Forget about perfection.