Drilling holes straight in wood

I am currently installing wall shelves using 40mm thick timber.
In recent times I have always had a problem drilling holes straight in thick wood, they always go at an angle.
Is there a jig somewhere whichenables holes to be drilled perfectly straight through timber so this is matched up on both sides?
Do not want a pedestal for the drill as no room available.
Thanks

hold the drill straight simples

Drill bits need to be sharp or they can wander, a square stood next to timber can help give a visual guide to a 90 degree angle.
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