There is a calculation for it that you need to follow very closely.
Firstly take note of all your clearences on the inlet and also exhaust side.
Once you've done that then you can see which are the problems.
The calculation is as follows:
N = T + (A - V)
T= Thickness of the old shim
A= Valve Clearence measured
N= Thickness of the new shim
V = desired valve clearence.
Your best using a digital Micrometer to measure the old shim thickness.
If you look at the shim buckets you will see edges that are lowered, thats so you can use a valve tool or a screwdriver if your careful to push down on the bucket and it releases the old shim, your best doing this with 2 folk and getting some sort of magnet too get the old one out.
Once you've done the calculation you take your result to Toyota and theyll make one as close to your calculation as possible, you can machine them down if there too big, that was the old way of doing it.
Like so:
http://cgi.ebay.co.uk/brand-new-halfords-professional-digital-vernier-/330459463280
I recently had to do this on one of my shims.