Calisthenics Unity

It's a rest after each set. (If there's 4 sets, you do: set 1, rest time, set 2, rest time, set 3, rest time, set 4, rest a little, drink some water or something, and go to the other exercise or muscle group).

I too sometimes feel like giving up because of motivation, but I never gave up, sometimes occasionally I rest 2-3 days if I really don't feel like working out, or sometimes I just push through it, cuz I know I'm not gonna die, I was bored of running but I never stopped it, I kept running every day. My calluses (the things that are created on our palms when we grip the bar) hurt so much, they tore and hurt so much but I kept going, so make sure you never give up no matter what. And any question you have, we're here to help you! :)

Can confirm.
Rest a little after each set, you might stretch your abs a little, take deep breaths. When you're done with all the sets you can leave your bench or wherever you are and have a longer rest to drink or smth but don't drink too much during working out.
Also, if you feel a bit tired or just feel like not working out, try to overcome this feeling and don't skip the work out. If you're feeling really tired (havent much sleep, dehydrated?) then of course don't work out, that's not safe.

Your abs will feel tightened/flexed (in tension, you know what I mean) after a workout - that's obvious. But that shouldn't last for many days, for example when I started with bodybuilding, my sets were too strong and I was always so dead after the workout, I literally couldn't move my arms for 1-2 weeks. That's obviously too much, you have to know the difference between a good tired feel after a workout and that feel when your arms are just broken because you trained too much.

It's fine if you don't do max reps/sets. I do it like this:
If I don't feel like I did good enough/could do better on a level (not only on reps&sets, but on form as well), I don't go to another one ahead, I either do the same one again or go 2-3 levels back.
If I'm sure I can't do the next level (certain exercise or something), I either do that level with an easier variation/progression of that exercise (for example if it's pistol squats, I do assisted pistol squats or something like that), and I count the level as failed, or I don't go to the next level till I feel strong enough for it (not very often but I do it sometimes).

Just listen to your body, if you feel strong enough, advance to a higher level, if you feel like you could have done better on that level, re-do it or something, and yes going back a level or two is not that bad (even though it says 3-4), we go back so we don't have the same level/workout every time (better for our muscles), instead of doing the same level till we advance to a higher one.