No best way , just lots of options, depends what you have available and how large the hole.?
1.If hollow behind I squirt in a little expanding foam, trim when set and filler to finish.
2.Glue in a small piece of card or timber or plasterboard across the rear and back fill.
3.Push in a piece or repair mesh and fill.
Many more options.
On the one hand I'd have said to fill the larger one wih some expanding foam, then cut and fill as foxhole said, I think id be tempted were it my house, to just use some easyfill to fill the entire lot in stages, allowing it to set between each. This way it's only one product to buy and faff with. The foam route would be quicker, mind, as it sets faster than easyfill does
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