Shelf turned into desk not at all sturdy

Hi,

My nephew loves to sketch and write, but didn't have a designated space to do so. They had a shelf that hadn't been used in some time so I decided to try and make him his own desk. It had 3 shelves so all I did was take out the middle shelf, but now it wobbles badly.

Does anyone have any ideas on what I could do to make it more sturdy?

Please help.

Thanks!

I didn't find the right solution from the Internet.

References:
www.diy-forczxcums.com/threads/shelf-turned-into-desk-not-at-all-sturdy.288988/
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