How hard is it to replace valve seals? or what? and what do u have to do?

 

either on the car or off the car it is somewhat difficult.

If it is on the car you would need to do the string trick or compress the cylinder, or fill the chamber with a rag. this is done so you dont drop the valve into the chamber.

you would need a valve spring compressor which can be purchased or made. you would need an in car valve spring compressor if you are doing it while the head is on the block.

I would not say it is the easiest of jobs.