You could do all this leveling by feel, or buy a spray-can of SEM 38203 Guide Coat (black). I've heard of people using old black paint (which can gum up sandpaper excessively), or using brush on black dust (which is more messy but sand off more easily). I got this because it was there and cheap: $6.
Below, you can see some typical block sanding in progress on the right side of the hood, looking from the fender (using a short block and 180 grit so you can see the different progress in one photo). On the far right of the picture, maybe the right-most 20%, you can see it is un-sanded. Just to the left of that, in the next 20%, you can see the "X" marks from me starting to cross-hatch as I sand a few strokes in each direction. (You always want to sand in an X pattern to keep from leaving grooves from the paper and to avoid the edges of your pad from digging in. There are loads of tutorials and videos on You-Tube about this.) Just to the right of the middle of the picture, you can see I'm taking the top off the rough texture, but there is still some substantial guide coat still left down in the bottom of the orange peel. This is also when you will start to notice high and low spots. The high spots get lighter, and the low spots stay dark. Keep sanding the whole panel evenly, but stop if you start to go into your etch primer, and stop before you go down to bare metal. On the left-hand side of the picture, you can see a few faint low spots that I was trying to sand down to, but there was one stubborn high-spot where I sanded down to the etch primer, which is clearly green. (Thank goodness for contrasting colors of primers. This would be a pain if they were both gray.) From here, you can do the wrong thing (avoid the high spot and just focus on the low spots, which leaves your panel uneven and with a high spot. Or you can hammer down the high spot. A great DIY trick I saw on a video was to take a thick solid metal spatula, hold it over the high spot, and gently hammer all around the high spot. The flat metal of the spatula distributes the force of the hammer, and keeps you from messing up the paint or metal. It also keeps you from hammering too far and ending up with a low spot. Spray on some more guide coat, and get back to sanding, hoping everything has evened out.
In this next picture, you can see a microfiber rag. You can buy a pack of 10 of these from Costco for about $10. It is a great deal. I got one from my buddy. As you block sand, you end up with a panel covered in dust. If you blow it off with compressed air, or your lungs, you end up with a dust filled room. If you take your microfiber cloth and wipe off the dust, it gives does a better job, faster, with less or no dust in the air, and you don't have to take off your mask to blow the dust away.
Here, the hood is done, and many of the spots where I sanded down to etch primer also had some metal showing. I decided to be prudent, and used some more of the spray-can etch primer on those spots to keep the metal from being exposed. (After cleaning the dust off them and wiping them down with grease and wax remover.) The spray can etch primer is dark gray.
I was able to finish the hood of the car, including the headlights and valance between the lights, and the roof of the car (not including sunroof), in about 3 hours. Here is me being proud of my first big block sanding session. The first of many.
Time spent today: 3 hours
Money spent today: $6 for guide-coat, $0 for microfiber towel.