Blanks? If you can get some scraps of maple, cherry, walnut, etc. that would make very nice wood to start with. I would cut them to 3/4" sq. with grain the long way, to start with. I found that the length is not important unless you want to fit them in a box.
I mark the center (where the grain meets) with a carpenter's pencil, and put a rubber band around each pair of blanks. Of course, if you are going to make slimlines out of the, you can drill them at this time, and even glue the tubes in. I don't think you should prepare too many blanks to start with, while you are experimenting with your first few pens. Until you learn to glue the tubes correctly you might 'experience' a few blowouts.
Which brings the point of drilling and glueing. PLEASE, remember this is a forum, and we all have our own ideas. What works for me does not always work for someone else ....
The brass tubes MUST be properly prepared. Brass tubes have a very slight grease coating from manufacture. This must be removed. The3 tube must be scuffed with sandpaper, I use 80 grit, to give the tube some 'tooth' for the glue to hold on to.
The hole, when properly drilled, should allow the tube to be inserted easily. Remember, you need space between the wood and the tube for the glue. This space is about the thickness of a sheet of typing paper.
I usually put glue on one half of the tube, and insert and twist from one end. Then I remove it, finish coating with glue, and insert it all the way from the other end. Unglued areas, especially at the ends, cause blowouts when turning the barrels.
Happy penturning !