Hmm, not sure exactly what kind of situation you're dealing with for the install.. there are several different kinds of belts, even in the shoulder/lap belt variety. Here is a pdf that goes over some of the different kinds of belts and how to get them to lock the belt part. https://fcs.tamu.edu/safety/passenger_safety/pdf/understanding_your_safety_belt.pdf
Some tips:
https://www.car-seat.org/showthread.php?t=28539
But if you really need help, post on that message board.. include all details (age/weight of child, make model and year of car, RF or FF, seatbelt type, specify a belt installation etc.. and include pictures of what you can get done on your own and they can help you). The ladies there are super nice and helpful.
Also, you can call your local fire department and see if they have a certified tech who can help you install it.