Player skin 
There are three ways of defining a player skin:
- Setting your player UUID (see here) to their Mojang UUID, clients by default retrieve the skin based on this value. This is done automatically by MojangAuth.init()
- Changing it in the PlayerSkinInitEventevent
- Using the method Player#setSkin(PlayerSkin)
How to retrieve skin data from Mojang 
Using PlayerSkin methods 
PlayerSkin offers some utils methods to retrieve a skin using simple information such as a Mojang UUID or a Minecraft username
java
PlayerSkin skinFromUUID = PlayerSkin.fromUuid(MOJANG_UUID_AS_STRING);
PlayerSkin skinFromUsername = PlayerSkin.fromUsername("Notch");WARNING
Those methods make direct requests to the Mojang API, it is recommended to cache the values.
Retrieve texture value & signature manually 
Most of what I will say is described here: https://wiki.vg/Mojang_API#Username_-.3E_UUID_at_time
You firstly need to get your Mojang UUID, which can be done by a request based on your username:
 GET https://api.mojang.com/users/profiles/minecraft/<username>Then, after getting your UUID:
 GET https://sessionserver.mojang.com/session/minecraft/profile/<uuid>?unsigned=falseYou'll get here both the texture value and the signature. Those values are used to create a PlayerSkin.
PlayerSkinInitEvent 
The event is called at the player connection and is used to define the skin to send to the player the first time. It is as simple as
java
GlobalEventHandler globalEventHandler = MinecraftServer.getGlobalEventHandler();
globalEventHandler.addListener(PlayerSkinInitEvent.class, event -> {
   PlayerSkin skin = new PlayerSkin(textureValue, signature);
   event.setSkin(skin);
});Player#setSkin 
java
PlayerSkin skin = new PlayerSkin(textureValue, signature);
player.setSkin(skin);