The question is already specific enough, and finding a tutorial for a new member is not the easiest thing.
Anyways, I assume you're using Mari0: SE beta. If you're not, then just say you are and I'll show you how to do it for 1.6.
Making a custom character can be challenging if you don't have much practice in graphic editing and coding, so if you know how to do that, this should not be too difficult. If you want a basic guide, you can find one here:viewtopic.php?p=91244#p91244
As for installing them, it is much simpler.
Open Mari0: SE, go to the mappack selection screen, press M
on your keyboard. Next, create a folder called characters
, and place your custom characters in there. Each will have a separate folder, so, for example, if you downloaded a Mega Man character, you would make a folder titled something like mega man
, and put the custom character files in there.
So, the layout should look like this, changing the character name respectively:
I hope this helped! :)