In terms of updating your avatar, you can set an avatar per character. In this post, I've elected to post as my character for the game, so you can see my character's avatar with my player avatar smaller in the upper corner.
In terms of etiquette and things like that, there are a few things that I've found to be pretty common throughout play-by-post games:
Character actions are posted in unadorned plain text.
The towering man draws the massive sword from his back, setting his stance against the upcoming rush of kobolds.
Your speech is done in bold text.
Dirty kobolds. Vile creatures. I would as soon bed a devil as I would to see a single of these vermin roam free. the sorcerer thinks to herself as she feels the wisps of poison start to coalesce in her hand.
I do information about the spell I am casting or the actual game mechanic I am using in red font. I also try to include a link to the spell in the post, just a quirk of mine so I can get to the spell details quicker.
OOC:
Casting silence.
1st 4/4 | 2nd 3/3 | 3rd 2/2
39/39 HP | 13 AC | Inspiration 0 | Wild Shape 2/2
Currently Prepared (
2 - [Heal in Wild Shape form - 2d8], Heat Metal, Moonbeam, Locate Animals or Plants
This lets me capture the mechanics I'm using that round, my spell slots available, my HP, AC, special features, etc, as well as my current prepped spell list (for casters only, obviously :) )
Looks like this in text form (remove the '/' character in the first tag to make it work):
[co/lor="red"]Casting silence.[/color]
[co/lor="blue"]1st 4/4 | 2nd 3/3 | 3rd 2/2[/color]
[co/lor="teal"]39/39 HP | 13 AC | Inspiration 0 | Wild Shape 2/2[/color]
[co/lor="tomato"]Currently Prepared (
2 - [Heal in Wild Shape form - 2d8], Heat Metal, Moonbeam, Locate Animals or Plants
Rogue Rapier Stab with sneak attack: 1d20+5, 1d8+3, 1d6.
This will let the DM grab your damage right away if it hits, or they can ignore it if it's a miss.
Spoilers are very useful for things like speaking in other languages (so other players can read it if they have the language or can skip it if they don't have it). Also useful from a DM perspective for things like perception rolls:
[spoiler="Perception 15+"] You see whats going on. [./spoiler]
With regards to the GamersPlane dice roller, there are some things to be aware of (examples included as rolls on this post):
The system doesn't currently allow for adding dice rolls together (such as 1d10+1d4). If you type in a roll like that in the dice roller, you will end up with 1d10+1 and a second roll of 1d4.
You can put multiple rolls in a single line, very helpful for advantaged rolls:
Advantage Attack Roll: 1d20+5, 1d20+5, 1d8+3
Reroll Aces is from other game systems where you "explode" the die when it gets its maximum value. So if you were to roll a 1d6 with reroll aces, and you got a 6, it would reroll the 6 and add the new dice to the total in addition to the 6. Example below.
This is all just stuff that I've seen and used throughout my various times playing and running games.
[quote=""] These were the rolls he made on the post as examples [/quote]
Rolls
Rogue Rapier Stab with sneak attack - (1d20+5, 1d8+3, 1d6)
1d20+5 - ( 15 ) + 5 = 20
1d8+3 - ( 4 ) + 3 = 7
1d6 - ( 5 ) = 5
Broken Adding Dice Together - (1d10+1, 1d4)
1d10+1 - ( 3 ) + 1 = 4
1d4 - ( 3 ) = 3
Advantage Attack Roll - (1d20+5, 1d20+5, 1d8+3)
1d20+5 - ( 7 ) + 5 = 12
1d20+5 - ( 2 ) + 5 = 7
1d8+3 - ( 3 ) + 3 = 6
Reroll Aces - (10d2, RA)
( [ 2, 1 ], 1, 1, 1, [ 2, 2, 1 ], 1, 1, [ 2, 2, 2, 2, 1 ], 1, [ 2, 1 ] ) = 26
You can be lvl 4 or 5. Your choice!