Work on a soft cloth or towel so that parts do not bounce if they fall.
Carefully half-undo the three grub screws on the front element rim and then lift it off. Never fully unscrew those grubs as they are too easy to lose and very tricky to get back in.
You'll probably find that the middle and front lens elements are stuck together and come out as one piece. It's because of the old grease they used which turns solid.
The two brass element rings are a little delicate and it's possible to crush the 'outer' middle section while extracting the 'inner' front element - with the result that you can't screw the front element back in again. Agfa used several variations in their design, and some are harder to grip than others, though the principle is the same.
First clean the exterior with Ronsonol and a cotton bud to take off any loose grease. You may be able to use a large needle to get between the two parts and pick out any loose grease from the seam. It's a good idea to make a small mark across the edge of both with a needle file, so you can then see if you are budging them at all.
Try separating them by hand. It's a normal thread direction. Any movement may be just one millimetre at the beginning. If they come apart by hand, great, you can carry on with cleaning them, carefully picking out every scrap of old grease from the threads with a fine needle and some Ronsonol and/or gun oil on a cotton bud.
If they don't come apart by hand then you will need to use heat. I set the surface of one element into a suitable vice (not too firmly, but enough) and blast the elements with a hairdryer on full, close up, until they get hot. You then need a suitable grip to start unscrewing the element. You can use a narrow strip of thick leather as a pad to prevent your wrench from scarring the brass.
The best tool to use is a pair of these grips made out of 2mm aluminium sheet (Photo courtesy of Scott Perry), as using a vice and wrench is really living dangerously. I've tried all sorts of soaking in the past, and it may help if they are ready to separate anyway, but it always comes down to heat from the hairdryer, and a positive and secure grip.
Then you can clean them up and insert the middle element back onto the shutter. You want to make sure the surfaces and the glass are completely clean - some Window spray and some lens papers should do it, finishing up with careful blows from an air-duster can.
Pro technicians will use a rubber stopper to grip the middle element and screw it into place. Sometimes the middle element has a groove cut into the edge which allows you to use a steel rule or knifeblade to get it tight onto the shutter. If it doesn't have a groove you could actually make a clean cut with a razor saw - just 0.5mm deep, however, finger-tight is usually enough if the surfaces are clean.
Incidentally, cutting a groove is a handy technique for any time you are trying to remove a middle element from the shutter that really won't budge.
Clean up the glass on the outer element and add a small amount of grease around the thread. It doesn't take much and you don't want to be cleaning off excess afterwards. I use a copper grease because I happen to have some, but most other types of mineral grease are probably fine.
The front element should rotate smoothly all the way round, in and out.
Before you refit the aluminium ring you need to collimate the lens. Most people use the infinity setting, but personally I always use the 2 metre or 6 feet mark as I want to be sure I can use the camera for portraits.
Focus setting is best accomplished with a small ground-glass placed against the film gate (tracing paper can be used, taped in place) and a magnifying loupe (an inverted 50mm lens from an SLR camera might do). If you need to, use a cable release to keep the shutter open, and set the aperture fully open.
Find a target to focus on at 2 metres from the film plane (not from the lens), ideally something with a sharp outline and under bright lighting, and look at it through the loupe against the ground glass.
Turn the bare front element backwards and forwards until you have the sharpest focus - and then take care not to move it again. Lift the front rim back onto the lens element with the 2-metre point lined up against the distance mark on the shutter body. Hold it in place while you tighten the grub screws.
Check the focus again against a suitable infinity target (I use a radio mast about 1 mile away) and at 2 metres.