Is there a guide on how to start doing Sekhemas or Trial of Chaos carries?

Try doing them for free first. Advertise in global, see how people behave and what they do. If you/they fail you have no obligations 

I know that sometimes the people joining need to have a minimum EHP of 800 and other times they don’t need that but why?

If you anticipate getting hit by zar then the honor damage will be dealt to all players in the party. Therefore having a baseline amount of honor is necessary for those people to not fail and get kicked out.

The formula for Boss HP is HP = Base HP * (Number of Players + 1) / 2. So, effectively, every extra person in your party adds 0.5x the Boss HP bar overall. So, make sure you have enough damage for the amount of people in the party.

In the unlikely event you are a Witchhunter using Sorcery Ward, you should be able to do Sekhemas carries without any honor issues. For other classes it will depend on if you can get to the boss without losing honor, and if you can beat him without losing honor. Honor is shared in the group, so if you get hit, they get hit, if you have lost honor before the fight they may just fall over dead as soon as they step inside - and randoms aren't clever enough to equip HP/ES gear before buying a carry.

For Sekhemas, get to the boss, open the portal to him, then portal out and go back to the Trial entrance to get people in. For Chaos I have no idea as I tried to help someone and they kept getting warped in and ultimately died, whether that was because they didn't listen when I told them to stay put at the start I'm not sure.

I would suggest watching what others do for free carries first. It helps to know when to tell people not to follow you.

They ask for people to have high hp in case they get hit

Join a few, see how they do it, copy