Haw do you time the engine up on a wr426 2002 model

There is no real way to change timing on YZ except change the cam itself to YZ450 cam or simular etc If your wanting to find out how to re install the cams you have already and cam chain then it is simple process but sounds complicated when its written
set piston to TDC (top dead centre) this can be done by spinning the fly around until you can see the (i) mark stamped on the flywheel line up with the marks on the inspection plug hole (engine case) good idea to probe the cylinder through the spark plug hole to confirm the flywheel mark is correct (I have seen several 426) where the TDC mark does not give accurate TDC on piston so worth second guessing this process.
once TDC is set do not allow piston to move from this point on - starting with the exhaust cam with no slack on the cam chain the dot stamped on the cam sprocket should align with the face of head with the cam lobes pointing outward roughly 45 degrees towards the front wheel when this in place you do the same with intake cam except the cam lobe will face opposite direction outward and the should be a dot aligning to the head
double check before you go further that when you put your finger in the chain tension hole and you tension the cam chain that when all slack is taken from the cam chain that your dots on both cams align with head face and that you still have piston at TDC if all checks out then this is timed set correct so torque all the cam caps back up install chain tensioner and the recheck all fly wheel marks are in correct position and piston is at TDC and the dot on each cam sprocket aligns to the head if it doesn't then re do the process don't be surprised if this takes a few attempts as it takes the best of us usually at least two attempts to get all marks to align correctly thats why best to check and double check before you bolt it all down then you can adjust just by lifting one cam out and moving one tooth either direction
You have little room to play with and when you start putting chain on the cam sprockets you will find very little extra chain length to play with you may need to wiggle tug and pull both cams together to get them back in each time but go slow and gentle they do go in they just need to be encouraged
I happy to elaborate on any point if you need - good luck