Since the USB host shield is designed to fit directly on top of arduino, how should I connect the pins for teensy?
Based on
this shield's schematic (but not having actually used it myself), I'd imagine you would need to connect digital pins 7 to 13 from Teensy to the digital pins on this shield. Of course GND needs to connect to GND. Keep these wires "short". A few inches is ok.
Teensy 3.5 follows the same pinouts as Arduino Uno for the SPI signals, so odds are good this shield will "just work" if you connect the signals directly (7 to 7, 8 to 8, and so on). In the comments, people mention issues with pin 7 on regular Arduino boards. You might need to add a few lines of code at the beginning of setup() to pulse pin 7 low, then keep it high while you actually use the USB Host Shield lib. Pins 11 to 13 will automatically be used for SPI. You might check whatever host shield code you're using to make sure it's defaulting to use of pins 8, 9 and 10 for those other signals.
This shield gets all its power from the VIN pin. My best guess is you could just connect VIN from Teensy 3.5 to VIN on this shield, at least if your USB devices don't need a lot of power.
There's no point connecting 5V, 3.3V, AREF or digital pins 0 to 6, since this shield doesn't use them.
This is all a bit of guesswork, based only on reviewing the info on Sparkfun's site and my previous experience using the host shield lib with other shields. I haven't actually used this Sparkfun one. You might need to adapt things a bit, but hopefully this starts you on the right general path?