The way we used to do it in the workshop was:

a) Dampen the veneer and press between sheets of clean paper for a few hours to flatten them if necessary. (Don't use plastic sheet - it will cause mould marks very quickly) I dare say you could flatten them with a steam iron, but I've never tried it.

b) Overlap the veneers and cut through both off a straight-edge, or, better, clamp between 2 boards and shoot the edges to make a crisp butt

c) Butt them and tape them together (sellotape works OK, but getting the residue off can be a pain)

d) Unless it is thin stuff fold them back a tad and run some glue in (PVA is fine), then glue the veneer to the core and clamp in a press. If it is fairly thin board then put backing veneer on the other side at the same time, otherwise the difference in moisture content can cause distortion

e) Gently does it with the sander!

If you are glueing it onto ply then don't forget that you are just adding another ply layer, so it is more stable if you put the grain at right angles to the previous layer. If it is properly glued then it can't shrink.
BTW how are you going to clamp it? Options without access to press are: for small jobs use loadsa clamps; for bigger jobs then a rockered board clamped at each end can do it; for DIY on a quayside in Spain - place the job between 2 thick smooth boards and park a car on it! Seriously