Does anyone know how? I can't figure it out.

Depends on what you want. RPG has categories for writing ability (free, casual and advanced), as well type (1x1, tabletop, arena and nation). You'd click where it says "X roleplay" (X being the category) and make a thread there. However, generally you would want to make an interest check before making the roleplay as to find people interested in your idea.

Also, welcome to the guild.

Always start with a list of ideas you have, then break them down into categories; 1x1, group, romance, adventure, sci-fi, etc. Then, do as Kuro stated above.

Just go to the category you're looking for, then click, "New Post". It will also have up to seven options you can choose from, depending on which idea(s) you share.

Best of luck!

Looks like everyone's answered your question, so I'll just say welcome.