Depends...
From the main page (or
your profile page, suggest refreshing first), click on the giveaway tab and ignore the list on the left side.
Note that if your on Android its hard to see the text unless your rotate your screen the long way or use the desktop version of the site.
On the right side you will see the options to create a giveaway, the rest should be pretty self explanatory, especially if you bought the bundle on indiegala (but I have never tried, someone else may be able to elaborate since I'm new to this) so long as you can see the entire page.
If its not a bundle from a known site go to the bottom area and you will see a box that says enter "Insert Steam..." or similar next to a button that says "Retrieve App Data" .
What you need to do is open the EXACT game page in your browser (Deluxe Edition etc.) if you want the right points and copy the URL from your browsers search bar, put that in the box next to "App Data" and click the button.
It then shows you a box next to the first one with your product name in it (or part of it) for your Steam key, insert the Key there and click "Select".
Note: Beware of using any key not for STEAM, some people will get angry and you'll get nothing if they down vote you.
If you do--at the least leave it in the description in detail (the very top box clearly labeled, regardless of how you do the giveaway)...and its still a risk you take.
At the bottom of the page it will show the game you chose above a few buttons:
Cancel - Remove the product from the list.
Calculate - Shows what you will gain for it.
Click Calculate if you wish and then click either:
Cancel - Do NOT put your game up
Create Giveaway - Complete the process.
After that you will find your own giveaway in the lists on the left side (usually the last option) under "In Progress" or "Completed"
Ok, that was way more complicated of a step by step than I expected, I should have linked you to the help page haha...but I hope this answers your question fully.
If its for a Gala Bundle and you have issues, I unfortunately can't help...but I assume its basically the same except there is no need for all the Steam information or entering a Key.
Any other questions, feel free to ask. Cheers.