There are 2 ways for initial practice of this asana :
with the belt/strap
with bend legs.
It is important you get even stretch on your legs and back. And some practitioners don’t feel anything in the back until they go forward far enough.
Paschimottanasana is about extending back. Usually back is strongest component here and overdoing hurts hamstrings, because they are weak. So the safest approach is to bend legs. Also one can use a pillow(s) under knees to get into more comfortable long hold, or above knees to support body.
Sit on the floor ( dandasana )
Bend legs and take feet with hands.
Elongate back (entire back) and feel extension.
Straighten legs to add more stretch to your back. You can extend your legs until you start to feel a little pain in hamstrings, then a little back and hold. NEVER push to a pain in your back.