Cams need to be in place to check clearance. Pretty straight forward job, time consuming though.
Start with a cold engine.Get the rocker cover off and turn engine to TDC on #1. From here you can check clearance on all valves on #1, intake on #2 and exhaust on #3. Turn the crank 360 and check all #4, #2 exhaust and #3 intake. Write down the clearances as you go.
We're loooking for around 10-11" on the intake side (think 6-10" is within spec) and around 14" on the exhaust side.
This is where the fun begins
You have an opened top end, skinned knuckles and a page full of random numbers...
We need to fit shims of the right size (thickness) to achieve clearance specs. Not all shims will need changing. You can get a nifty tool from EBay to pull em out with cams in-situ or pull the cams out, sorta depends on the job/day.
Any valves not in spec, remove the shim and measure the thickness with a digital caliper. Now, say your #1 front ex' valve clearance was 12". You need a shim 2" thicker to replace it with.
Shims are readily available from Toyota in about 14 sizes. Precision shims make em too. Me being me, I went to the local self serve wreckers and pulled shims from low/mid Km 5vz camrys heheh.
There's a bit more to it but the suns out and the grandsons are harassing me to go do stuff, sure a few others can go into more detail for you mate.