Cut your log to size...i cut mine to 10"
Sand your log...but don't remove all the bark...the bark gives a nice look.
Flatten one side by cutting 1/4" or less from one side with a jigsaw...or you could cut it off with a knife and sand it
sand it till it's very flat that when you keep it down and push it ...it'll not roll or even move.
Mark you log for the holes.
I marked 2" from either side...eyed the center and nailed it slightly.
then i put a band on both...and marked 2 more holes with nails in the band area...that way they were straight
My drill could take a maximum of 13mm drill bit...the test tube was wider...hmmm
well...i drilled the hole about 3/4" deep...widened it with a rotary sanding band.
If your log is not the same width all around ...you might want to drill a bit more in some holes to make all the tubes the same height.