Starting with your arrows, I think they are fine. They overall look flowing, although there are some places where you could overlap them more. Do not be afraid of verlapping them, that helps the flowness of the arrow overall.

One thing I tend to recommend is to shrink the farther end more, so the illusion of 3d is stronger.

Now, with your sausages, there are some that kind of stray from the one requested (check these notes: https://drawabox.com/lesson/2/5/simplesausage), so be careful with that. Drawing them like that can difficult our constructions in the future.

I noticed that many of your ellipses were messy, and remarked more than two times. You just need to remark two times (ot three at best), otherwise it'll look weird. The ellipse degree overall looks acceptable.

In the ones with contour curves there were some that weren't added properly; they weren't added thinking on how they would wrap in the surface that they are covering. That can ruin the illusion of 3d, therefore you must always be aware of how the things you add to your 3d forms sit in them.

I also think that just three or four contour lines suffice, that's because too many that aren't well placed won't do the work you are trying to achieve: that of conveying the illusion of 3d.

I see that in your textures you focused on the cast shadows. That's excellent! But still there are some issues:

1.- In the analysis there's a predominant of form shadows, recall that you should focus on the shadows that are casted by other things.

2.- In the dissections, there are some textures where you filled them with completely black. You don't paint in drawabox, right? Just because the color of your pen and the texture match, that doesn't mean you have to fill it (like filing an animal eye just because it's black), that's why you rather use black for the shadows, to maintain solidity. There are also some where you outlined lines too thick -and again- just use cast shadows.

I think you could understand the relationship that exists between forms in the organic intersections, and that's good. Intersections are tricky, but I have a tip: when drawing one, follow mainly the surface of the one that's being intersected, but not forgetting the one that's intersecting. Always think on how they relate when you draw the intersections; how they cut through the other.

However, there are some boxes that are kinda large; always draw them as equal in sides as possible.

I think you could overall understand as well the relationship between the sausages in the organic intersections, but there are some that look off because they are...beneath the big sausage, or sitting as like they were humans (one in the first page); that's agaisnt gravity, so avoid that. The only problem was, of course, the distorted sausages.

So overall I encourage you to continue practicing 1)your ellipses and 2)your sausages.

About when you should do the cylinder challenge, it's recommended to do it before lesson 6. Meaning, finish lesson 5, and then do the challenge. Although there's people that do it little by little on the go. Either way, just make sure to do it before lesson 6, but don't worry if you don't do it before lesson 5 :)