If they're slightly bowed right out of the box, I lay it against a wall and run my hand or a dowel down the ends of the ties. That seems to give the ties a nudge around in the rails and can straighten things out. I glue it down with Elmer's, putting heavy things on it while it dries. For dead straight, You can lay a long metal ruler against one side of it, or as mentioned above, set something in between the rails. Fastracks makes a lasercut stick that will hold your track straight. The sections interlock together and you can make them as long as you want.
https://www.handlaidtrack.com/sw-o-s-10
If the ends aren't even, the micromark track nipper (the Xuron one also works, It makes, IMO a cleaner cut but its not as durable on code 148 track so treat it like a wear item), or a dremel will square it up but having staggered joints isnt a bad thing, just adjust one piece to fit the next.
