This guide is based on multiple Lexus Models.
Disclaimer: As always, this is a guide only and I accept no responsibility for any damage or injury that maybe sustained through following these guidelines.
Tools I would recommend (excluding wheel removal):
1. Breaker Bar (only required if changing discs)
2. 1/2 inch drive rachet (optional)
3. Large screwdriver or lever
4. Copper grease
5. Piston retraction tool (g-clamp or anything similar for pushing the pistons back).
6. 17mm socket (not pictured)
OK, now down to the nitty gritty. I will do the whole thing including wheel removal just incase someone doesn't know how to do it.
1. Loosen (DO NOT REMOVE) wheel nuts before jacking up the car.
2. Open the bonnet and remove brake fluid resevoir cap
3. Jack up the car and support on axle stands.
4. Remove wheel nuts and remove wheel to expose the brakes
5. We will deal with the pad change first.
There is a small split pin at the rear end of the pad retaining pin, remove it.
6. There is an anti rattle clip to be removed next in two stages, push/slide it up first, the pivot it out from the top. it can then be withdrawn.
7. The retaining pin can now be pulled out towards you and removed.
8. With the screwdriver, just pry the pads away from the disc slightly (doesn't need to be much), so that the two pads can be withdrawn.
9. Retain the anti-squeel shims incase you need to re-use them.
If all you are doing is the pads, you can just re-assemble in the reverse order of the above steps. Don't forget to apply copper grease to the backs of the pads and between the each anti-squeel shim. IMPORTANT: Do not get any grease onto the face of the pad which touches the disc! Jump to step 16 for the other steps.
10. If you are changing the discs as well, then continue with the tutorial.
The caliper needs to be removed. There are two 17mm bolts holding the caliper to the hub.
11. Once removed (a breaker bar is very usefull for this), support the caliper on an axle stand or tie it up to the coil spring with some string or wire. Do not let it hang by the brake line.
12. ensure the handbrake is OFF (if you only have one rear wheel jacked up, you will need to put the gearbox in neutral also), as you will need to rotate the disc so that the inspection hole is at the six o'clock position. remove the rubber bung if present.
13. If you are lucky, the disc may pull straight off. If not, you need to back off the adjustment on the hand brake shoes to give enough clearance for the disc to come off.
The adjuster is on the bottom edge of the shoes, and is accesible through the insepction hole that you have already aligned to it.
Using screwdriver through the hole, rotate the top of the adjuster towards you.
I have done a couple of pictures to help visualise what it is you are trying to do, first pic is with the disc already removed to show the screwdriver going through the hub.
next is two superimposed pics of disc on and off.
lastly, from underneath with disc removed showing the screwdriver tip on the adjuster. It also shows what the adjuster looks like so that you know what you are looking for through the hole.
Anyway, hopefully using a combo of those pics, you get the idea.
14. Next are a few pics of the shoes from different angles, just in case you are taking these off for some reason. Shoe wear is tiny if none at all. As the shoes are never on when the wheels are turning, they don't wear down. Don't look at the amount of material left on them as a guage to replacement, brand new ones have a tiny amount on as well.
15. OK. So now the disc is off, and you can put the new disc on. Make sure you align the inspection hole with the hole in the hub. If you forget this, you won't be able to 1) back-off the handbrake shoes next time if you need to, and 2) you wont be able to adjust the handbrake at the end of this change process.
16. Using either 1) caliper piston retraction tool, or 2) levers or 3) clamps, push the pistons back into the caliper.
Refit the caliper to the hub using its two fixing bolts.
17. Apply copper grease to the back of your pads, and refit anti-squeel shims if you need them. IMPORTANT: Do not get any grease onto the face of the pad which touches the disc!
.
18. The rest of the refit is just the reverse of steps 5 to 9.
On the GS at least, the handbrake is not self adjusting. Once the disc is back on, you need to adjust the handbrake with a screwdriver through the inspection hole in the disc. Push the top of the adjuster away from you until it wont turn anymore (don't force it). If you try and turn the disc by hand now, it shouldn't turn. Now back the adjuster off slightly towards you, try and turn the disc, if it wont turn still, keep turning the adjuster back until you can turn the disc. Thats it. Easy.
When done, admire your shiny new discs 
A note here, which I have seen mentioned before and happened to me, the lip on the backing plate of the handbrake shoes was catching on my new discs. I had to use an angle grinder to grind the lip down on the backing plate until it cleared and didn't catch anymore. A bit of a pain it has to be said. I would imagine that you won't hit this problem with OEM discs, and it may well be hit and miss if you do have the problem with copy aftermarket parts
