Hello, Ludumberg here, ongoing workshop creator. Please be in mind that english isn't my native language so I will do some grammar mistakes. If you find some mistakes feel free to post it down here.

I've been receiving many friend requests of how to create a workshop item in Team Fortress 2 and even in CS:GO. In this guide I will cover step by step the whole process begining from a rought idea to the final step of publishing your own item. I will not go much into how to use a program. I hope you all know how to use a 3D program and graphic program. If not you will not be able to make an item. There are plenty of guides on how to use a specific program.
The programs I always use for creating a workshop item:

- Maxon Cinema 4D
A great 3D modeling software I have been using for 6 years.

- Blender
A free 3D program. I just use blender because there is no option in Cinema 4D of exporting the model to a .smd file, which is used for importing the model in the importer. And for creating an UV layout.

- zBrush
I just use this program for creating a highly detailed model for its texture. Like folds etc.

- Adobe Photoshop
The standard graphic program. No need for further explanation.

In your daily life nothing is created without having an idea. The very first step of anything in life is to think through your idea in your head. The one and only rule of that is: Do not force your imagination to come up with something. You let time and your environment be your inspiration. The inspiration for creating the long sleeved youth was when I walked home at a summer day, while wearing my hoodie like everyday. But that one day I looked up the mirror and *bing* I had this idea: "This hoodie will look perfect on the scout". And so it was created.

So you now have a rought idea. But you don't know how it will look like. The easiest way is to have your idea in front of your eyes. Just like in math, the easiest way to solve a mathematical calculation is to write it down on paper. And so it's with your idea. Draw it down, take a piece of paper and just draw what you have in mind.
Draw every single detail, maybe even color it. I sometimes use photoshop and draw a picture of what I have in my mind.

So, now that you know, what you want to have, let's begin in modeling. Open up your desired 3D modeling tool and start modeling your item on your class. I highly recommend looking at the Steam Workshop and the TF2 Wiki because the artstyle of TF2 is one of it's kind. At this point it's important that you know how you use your programs. Again in this guide I just explain you the process of rough idea to final publishing.
The items have a high quality standard. There are people who think putting a something like a cube that looks like a rubik cube onto the head of heavy is enough to make him sucssesful. You always have to ask yourself "Do I really want something like this in the game. Do I really want to wear my own creation. And Does everyone else think it's an original idea?" Some of these question can be answered. Simply ask your friend what the think of your creation. Ask them do they like it. You can also ask in the internet. Facepunch[facepunch.com] is a really great site where many maaaany creators discuss of their newest creations. Simply make some screenshots and ask them, do they like it and what should you improve.

Since I don't make a tutorial I still wanted to give you some help. These are great guides created by the community to help you how you properly create items:
HellJumper's NEW TF2 Modeling Tutorial
Jigglebone Tutorial for Blender
Facial Flexes Tutorial for Blender, 3ds Max and Maya
Creating Backpack Icons

As said the artstyle of Team Fortress is very special and much different than many other games. The textures consist of just some basic color schemes. And textures try to have as little detail as possible. The texture base consist of Ambient Occlusion, a prerendered shadow and light map, and some base colors.
Textures also have some rectangular brush strokes to give it a little more detail:
As you already know TF2 has two big base colors: Red and Blue. These colors are used to differenciate between the two teams.
For better explanation please visit this guide by Swizzle:
Texturing for Team Fortress 2: A Short Guide[polycount.com]

To create some good textures, you need the right tools. But fear not, I have provided you with the right tools for your texturing:

Team Fortress 2 Swatch collection[www.deviantart.com]
This swatch pallet for photoshop has over 250 individual colors found in TF2. Perfect for finding the right red tone or for creating some good metal.
Dutchman's TF2 Brushes[www.deviantart.com]
This is a collection of TF2 styled brushed packed into one brush set for Photoshop.

You have created your model, you have created a texture, you have exported it and you are ready to upload it to the workshop. But since the community can't just see your actual cosmetic in the game you have to make pictures. The most important thing is the thumbnail. That's the very first picture they will see in the workshop. A better thumbnail means more people get attracted and view your item.
Almost everyone uses Source Filmmaker. Why? because it's free, easy and it's also the tool Valve uses to create their promotional pictures and videos.
Source Filmmaker
But let's say you have no clue how to use SFM. There are plenty of videos on Youtube how to use SFM. Here are some of the basic tutorials:
Official Source Filmmaker Tutorial Playlist
Pte Jack's Videos

A good thumbnail shows the item and the class. Try to pose your character in the way you like. Remember: Less equals more. Try not to overfill your thumbnail with hundreds of effects. Use a map that fits the theme of your cosmetic and use the right lightning. You can also look up in the SFM Workshop.

To show your item you have to provide some actual screenshots of the item. I mostly use the Model Viewer of the Source SDK. It's fast and simple.
Source SDK
But since this version of the model viewer is buggy i recommend you using the TFVM. It's much more user friendly and you can even create a paint chart of your item in just some simple clicks.
TFMV

The best way is to create some pictures of your item in different poses from different sides. Maybe even make a chart where you combine your item with already existing ones.

Now you have published your very own item in the workshop. But let's say you want to go one step further. Let's say you want to publish it as a mod or as a hex for gmod?
Well the mod part is easy. There is a easy tool that let's you select many of the existing items in the game to be replaced. I use ItemToast. A very simple to use tool. You can of course publish it as a mod for the community.
Gamebanana[gamebanana.com] is one of the oldest modding community site. You can easily upload your mod for everyone. The community can rate your item and give you some feedback.
The hexxing part is a bit more different. Since I don't know how I hex stuff, my friend does all that stuff. Merkitzi Incorperated is a group where you can request items to be hexxed in gmod.

Again, English isn't my native language, and I would really appreciate it if you can help me improve my guide. I really appreciate that you took the time to read my guide. And thank you for your support. At this point I am done with my part. It's now your time. Get creative.
And never forget: Practice makes perfect.