Put the probe on the outside of the tank between the bottom of the tank and the heat mat. The top of the substrate will be a little cooler but that's ok as if the snake wants more warmth, he will burrow down or push the substrate out of the way. My little girl does that, she pushes the substrate aside inside her hide and just lays on the glass which I keep around 93-95 degrees depending on time of day. Cool side is just kept at 80 degrees. She bounces back and forth. I just wouldn't let the glass get over 100 degrees though. Temp guns also make it easy to check substrate and temps.
The tstat probe should be between the uth and the bottom of the cage directly on the uth.
The thermometer probe can go where ever you want to monitor a temp. Usually it is placed in the tank under the substrate to measure the hottest temp the snake could reach in the cage.
KMG
0.1 BP 1.1 Blood Python 1.0 Brazilian Rainbow Boa 1.0 Aru Green Tree Python
0.1 Emerald Tree Boa 0.1 Dumeril Boa 0.1 Carpet Python 0.1 Central American Boa
0.1 Brooks Kingsnake 0.1 Speckled Kingsnake 1.0 Western Hognose
0.1 Blonde Madagascar Hognose 1.0 Columbian Boa
Mine actually seems to like 95f although I do think it is a couple degrees hotter at the probe since the the probe is kind of fat and doesn't lay completely flush between the heat tape and the tank bottom so it creates a little pocket of air which I think warms up more. When I temp gun the tape, its usually about 1 degree cooler. Your snake will tell you also if they don't like it. If your snake never sits on the hot side, then its probably too hot. If it never sits on the cool side, then its probably too cool. If they go back and forth, then you got it just right......reminds of Goldilocks and the 3 bears