I want to make shelves to safe space inside my cabinets.
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You need to add at least 3/4" x 3/4" trim edge for your shelf to rest on, where you want your shelf. Screw and glue in place. Use 1/2" plywood for shelves no wider than 32".
Here's some simple and unique ideas
KV makes all kinds of shelving hardware that can be found at all hardware & DIY stores.
Measure the width then drill holes in the sides to put pegs to hold the shelf’s in then cut wood to fit the width and lay it in the cabinet on the pegs
I agree with Kelli on this, BUT a lot of cabinets already have little holes on the side walls inside the cabinet. If yours does, all you need is some shelf pins and cut shelving board. Both are available at any hardware or home improvement store
Lowe's makes shelving board that is pre-cut!
Otherwise fix timber runners along each side of cabinet and lay your shelf on top.