Do you mean add a shelf to your cabinet? If so, here you go:
You might try some cabinet clips like this: https://amzn.to/2SPtvWF
and then just cut your wood to fit.
Here's a link that gives you the steps for adding a shelf:
Something like this? https://www.frugalfamilytimes.com/2015/10/kitchen-update-for-way-less-cash-how-to.html
If you’ve holes for shelf pins, then it’s a matter of having a piece of melamine-faced pre-made shelving, ripped down on a panel saw or table saw.
That’s free at Home Depot & Lowe’s
Here's a good tutorial on adding shelves to your cabinet: https://www.youtube.com/watch?v=IwYe1KjWpnM
How about floating shelves? https://www.charlestoncrafted.com/how-to-build-and-install-floating-shelves/
You could just leave the door off. Remove the front facing and paint it.
You could use hidden brackets or standard brackets or fancy brackets. Check out your DIY to see what's on offer. Make your choice and cut your timber to size.
Fix the brackets - If brick walls - Drill - Plug and screw. If Timber - find a Batten and screw to that.....Place your shelf, fix in place and there you have it.. Best wishes!
You need a few pieces of scrap wood to make slats on the side of the cabinet. Screw them in the sides. Measure the inside, get a piece of plywood cut down at Lowes or Home Depot and put on top of the slats. Wala a shelf. Or you can buy premade shelves at the hardware store.