Loosen the walking belt
bolt the motor on tightly onto the frame
put the motor belt onto the motor pulley
Then use a small flat head screwdriver on the ribbed side of the motor belt but between the plastic part of the front roller pulley
With the screwdriver in place turn the motor pulley and keep turning until you can pry the belt onto the front roller pulley and then remove the screwdriver.
This is not easy and takes some time. I have been repairing exercise equipment for over 20 years and this takes me awhile to replace.
Don't worry about the motor belt being to tight the Flextronic belt expand and it needs to be tight so the walking belt will not slip.