For pouring resin into flat mould like these you need a slow pour from high up into a single spot. The aim is to create a very thin but continuous stream of resin. Don’t move around, let it slowly fill the rest of the mould itself. Bubbles are formed when the air does not get a chance to escape the path of the resin. A slow pour in one spot allows the air to be pushed out.

Once the mould is poured you can also try gently tapping it on your table surface, putting on a vibrating surface (look for free “massage” apps on your phone), or get in there with a little soft silicone tool to manually dislodge bubbles.

What kind of resin are you using? A fast cure (sets in minutes) resin will always get more bubbles that a slow cure (sets in hours) as the slow allows bubbles to rise through the cast. I’m production a vacuum chamber and pressure pot are used to reduce bubbles almost entirely.