Matching new oak floor planks to old planks
A picture of the area would have been good to see how the ends tie into the existing but here goes. 30 sq. ft. of floor is a lot to do. You may want to rent a random orbit floor sander to do the bulk of it, then detail the edges where it meets the existing.  When you write that the "planks aren't quite flat meaning they are higher than the rest of the existing floor, you can carefully mask off the existing floor around the new wood with 2" wide blue painters tape and sand the floor down to where the hand sander touches the tape, but not cutting through it. Of course if you do you will damage the finish of the existing floor. Start with 60 or 80 grit first and when the sander first touches the tape, change to the next finer grit. If it was 60 grit, change to 80, if it was 80 change to 100. Then take a regular #2 pencil and mark across the freshly sanded planks so  there are lines all over the place. Sand with that next grit until the pencil marks are gone. This will help you keep track of where and how well you have an area sanded. You may have touched the painters tape again, but as long as it hasn't torn through, that is ok. Mark the wood again with the pencil, and sand again with 120 grit, that is the last cut.

You will need a piece or pieces of the floor to test the stain on to get a good match. then stain the bare wood carefully and wipe it down with a rag to get the excess off the surface. Actually wipe the surface anytime you stain, even if there is no excess, there is always a residue that is naturally left in the process that needs to be removed before sealing. Read the directions on the stain for the time between stain and finish coats. Sand lightly (220G) after the first clear coat, then remove the tape, vacuum and dust thoroughly then apply the second coat of finish. cut it in carefully. Make sure to match the sheen of the existing floor.