Step 1
make your inventory public
step 2
add your trade url to your profile page, that means your profile page should not be private if you want people to send you a steam trade offer - how to find your steam trade url: on steam chat you can click the send special option looks like a paper clip button send your trade url to a trading group chat or inactive friend or something, you can also find the url on your inventory, then trade offers, then who can send me trade offers, then third-party sites, i also got this url https://steamcommunity.com/my/tradeoffers/privacy/ from https://help.steampowered.com/en/wizard/HelpWithEconTrading and this other url also requires login http://steamcommunity.com/my/tradeoffers/privacy#trade_offer_access_url

Create a trading topic at the game's community hub, specifically on the trading forum, some games lack a trading forum and some have additional rules about the title of your topic and stuff, please read the pinned topic at the correct forum if there is any. Edit: Seems "Inventory Privacy must be set to Public to receive Trade Offers from trading topics.". Reminder: These topics last for up to two weeks after the last reply and they used to be sorted by last reply, which has not been the case anymore for a while so people 'bumping' their trading topics are only keeping them open at the bottom of the topic list, which is sorted by date of creation instead. Other places, like steam groups, may or may not organize their topic list by last reply sent.

Authenticate your account with 2 factors, enable steam guard on your personal mobile device etc it takes 7 or 8 days for this to kick in and let you do instant trades but you gotta remain sorta active on steam idk how much

Please inform if anything changes so i update this, here's info that may save you from getting scammed, etc, remember, only steam items are allowed to go into a steam trade, disbelieve messages people add to their offers or promises they make that you'll get stuff after you accept weird trades that don't benefit you much by themselves, only accept trades that are directly giving you good items, remain in steam to do steam related things, if someone casually wants you to go elsewhere that is not steam to do something related to steam it can be a scam (i have fallen to this a while back), some people fake items, they find stuff that look similar and try to push, some go as far as canceling a correct offer and sending a new incorrect offer with incorrect item that looks nearly identical to the correct item of the other offer, there is a lot, but, otherwise, trading on steam can be pretty cool - official urls - 15 day hold: https://help.steampowered.com/en/faqs/view/34A1-EA3F-83ED-54AB
Recommended Trading Practices: https://help.steampowered.com/en/faqs/view/18A5-167F-C27B-64A0
Scam FAQ (Confidence Scams and Trade Scams) Scam: I have been reported and will be banned (etc): https://help.steampowered.com/en/faqs/view/70E6-991B-233B-A37B
Steam Trade Offers: https://help.steampowered.com/en/faqs/view/1115-91C5-050C-1D60
Trading and Market Restrictions: https://help.steampowered.com/en/faqs/view/451E-96B3-D194-50FC
this guide was possible thanks to https://steamcommunity.com/sharedfiles/filedetails/?id=1103622431