I'm making a scratch game too! That's pretty cool. The tool I use is TurboWarp, since it has a bunch of features and lots of people use it. What I would do is download your project, go to turbowarp.org, click the "File" dropdown, select "Load from your computer" and then select your project. Then you click the File dropdown again, select "Package project", and then it'll open up some settings. Usually I keep all the default settings, but you'll definitely want to change the Player Options part (which changes stuff like the loading screen, website title & icon, etc). You can also change the other settings for your own preferences, like framerate, mouse cursors, green flags, stop buttons, and other stuff. Then, at the bottom of the page, you can click "Preview" to preview & test out the project, and then you click "Package" to download the file. Then you can just go to the game jam submit page and upload the file there to submit it. I'm sure there are many other ways to do it, but that's just my method. Hope this helps!
(Plus, you can use the "Advanced" dropdown to change the screen ratio and some other stuff. It's not necessary though)