To create the holes for the burner(s), check out the boolean modifer; it basically can cut out the shape of an object (e. g. a cylinder) of your mesh. Additional info can be found on youtube or here.
Also play around with the bevel operator: select one or more edges in editmode, press STRG + B (release the keys) and move your mouse. The tool will round your edge(s), with the mousewheel you can add some subdivision to the “rounded edge”.
Use the knife tool, asign it to your mouse like religion, cut it out with k if i recall, and add to cylinders(no make it one cylinder, and use the mirror modifier.) model your cylinder, use reference.
Just get the out side or the tip of the jet engines nozzle of google images; load up a image inside blender trace over it with each vertices.
you’ll want to merge each cylinder to your mesh stitch it, merge it. cut the remaining cylinder you wont need the inside cylinder.