Put a halogen work light on the floor shining up onto work? If your booths are like the one at our school you can hardly see your hand in front of your face.

Use one hand... The other to balence on something.

For a beginner it's more easy to go from one side to the other than going toward or away from you.

Head not directly underneath... Just off to the side enough that slag drops somewhere other than your face.

You can use a little higher current than the verts most of the time, this will help make a big puddle that is easier to see.

Same as flat just upside down

Johnny Walker hit the nail on the head. Weld it like you were welding flat, just flip everything. Get a better view of the weld zone.  If your 6'5" you might have to squat down or get on your knees.
Alot of guys just starting on overhead stand about eye level to the plate or just slighly below. This makes it difficult to see anything. Imagine welding a flat stringer with your haed eye level to the plate. You'd probablly have the same problems.

Have you cleaned or replaced your filter plate or clear lens? They get dirty or scratched  little by little and restrict vision.

I agree learning to position your body, heads, and hands, so you can see, be balance and comfortable, and hands free to manipulate the rod is important in learning to weld.

And body position for overhead can mean the difference in getting burned up or not from falling slag and metal. Sometimes in a cramped hole or corner you have to lay right under the weld. Thats time to break out the leathers.

Floyd

When I did my unlimited I just sat in chair. Took about 3 hours to weld out and seems like it was 28, 1/8" 7018. I put on the leather coat, old beat up Jackson hood and got right under it. Went left to right on my plate. After the root and hot pass the slag would just peel off by itself and fall on me. Did not get burned up or anything, hood got a little beat but nothing major. My rod angle was centered in the groove and had about 5 degrees tilt towards the direction of travel, meaning my hand was forward of the weld pool. Seems I ran my amps about 120-130. I actually slouched down in the chair and raised my arms up so they were not being supported by my body. Figured I did not want to support them, cheat myself in case I needed to do this on a job where I would actually have to stand up. I wanted to be under the plate, gave me a clear view of the weld pool and when your stacking 28 stringers on top of each other and have to contend with a consistent appearance at the top being under it lets you see your highs and lows early. After the first three passes the weld would practically self clean. I could peel off large sections or whole stringer passes with one swipe of the brush, grab a new rod, check the weld for variations and then line up the rod, flip the hood and go again.

Seems I would start out on the left a bit slower and by the time I reached the other side I had sped up a bit leaving the right side a bit lower. So each pass was evaluated while cleaning, reloading to see what I needed to do for the next. I'd pick a point and slow down a bit and stack a little more metal to bring the stringer back into an even distance on the beveled edge. This way when I got to the last stringer and pulled the plate down I had a nice consistent stringer cap with a very consistent height off the top of the plate. My suggestion, thow on the leathers, button them up to your neck, good beannie hat and a old train wreck hood. Get right under the plate and go for it. Just how I would do it but that's me.