It's not the simplest thing ever but I'll make sure you can't go wrong. Just follow the instructions! This guide is for Windows but you can also use OS X or Linux. You'll know what to change to these instructions.

1. Create a "workshop" folder. Anywhere is fine. For the sake of simplicity, I recommend C:\workshop.
2. Inside "workshop", create a new folder for your new item. Give it the same name as your item if possible... preferably without spaces or special characters. I'll call it "itemName" for the rest of the guide.
3. Put the content of your workshop item inside "itemName". If it's a map, then it should look like itemName\itemName.pk3. If it's a script file, it should look like itemName\baseq3\scripts\itemName.factories . I'm no expert on this field so you'll have to figure it out yourself. As long as everything is inside the "itemName" you did it well for the "publishing" part of it.
4. Choose some image to represent your item. To quote Steam, "Image should be square, and at least 195px by 195px". Keep the same extension. Call it whatever you want. I'll refer to it as "itemName.jpg" in the future. Put it in the "workshop" directory.
5. Create two new files inside "workshop". Call the first "itemName.vdf" and the second "itemName.bat". Make sure it's not "itemName.vdf.txt" or "itemName.bat.txt". You may have to change your folder options with whichever version of Windows you use to make sure it displays extentions of known file types.
6. Edit itemName.vdf. It should be empty... but not for long. Paste that :
7. Change the different fields (contentfolder, previewfile, title and description) as needed and save. publishedfileid must stay at 0.
8. Edit itemName.bat. It should be empty... but not for long. Paste that :
9. Replace USERNAME with your Steam username. Replace "itemName.vdf" by your actual vdf file's name. Keep the ..\ there, though! Don't change anything else.
10. Download SteamCMD : https://developer.valvesoftware.com/wiki/SteamCMD#Windows
11. Unzip it and move the resulting "steamcmd" folder inside "workshop".
12. Double-click on itemName.bat. Input your password. You will be logged out of your Steam client. If you did everything correctly and Steam isn't down, you will see your item being uploaded. "Job Finished !!" will appear when it's over. Make sure there are no errors above that. You just have to press a key in order to exit the prompt.

Items may take a little while to appear once they've been published for the first time. If everything went well, "publishedfileid" in your VDF file will have changed from 0 to something else. It's your workshop item id. Don't ever change it. You may want to modify other fields in this file if you want to update your item in the future though, like the description or the changenote. Otherwise you only need to change the files you wish to modify and run the script again. You can use steam's UI on your workshop item's page to add videos and other things. I'll let you figure those out yourself!

Happy modding!