Casual is good. I'll give more advice to you as requested or if I feel a good opportunity arises to do so. As for right now, advice going into this is the basics: 
1. Don't control or automatically hit/kill the other person's character. If you post an attack say something like "He swung the blade towards his opponent's head with a violent sneer on his face". You cannot say "His sword slashed through his head" in your post because that's called god modding. In my post if I know I've been outdone and can't dodge it then I will say "he wasn't able to dodge in time, flinching as the metal moved unstoppably closer". Only then when it's clear that the opponent isn't dodging can you take your hit in your next post, doing the damage that would reasonably occur. 
2. Our posts will go back and forth by way of each post having a response to their opponent's actions then taking an action of their own. For example, if I was able to dodge your sword swing I'd say "he ducked low underneath the blade and then lashed out with his fist towards his stomach". Then it would be your turn to respond and take an action of your own, etc.

Feel free to object to any move that seems unfair. I might have just missed something or underestimated something. For example, I could say my character ducks below the sword, but if you give a reason why he couldn't move that fast in such an example then the post can be edited accordingly.

What character are you planning on using? I like making my own characters but you can use an already made character if you really desire. Make them as powerful or weak as you want as long as you get some good practice going on.