Why look after your silk?
Maintaining synthetic silk has several advantages:
-It already helps to extend its lifespan. A good thing from an ecological point of view, and a good thing from a financial point of view too, when you consider the prices some of them fetch.
-It also greatly improves your efficiency at the water's edge. A line that floats high in the water will be much more discreet when "snatching". On wild fish this really makes a big difference. It will also make it easier to make certain casts, such as the rolled cast or all the derivatives of the Spey cast. Here again, a high-floating, gliding line helps a lot. A well-maintained line will glide better and allow you to cast further and/or make fewer false casts. You'll gain in fishing distance and/or discretion.
How to care for your silk?
I'm going to give you my method, which I find simple, effective and quick to implement. I only use "large arbor" reels, which means that the line is wound on a fairly large diameter support (about 80 mm in diameter).

So I don't bother winding my line onto a larger-diameter backing and leave it on my reel spool. If you use a reel with a small hub, I advise you to use a wider support during the truce to prevent the line from retaining its memory and "corkscrewing". The smaller the diameter, the greater the risk of micro-cracks on the line.
I start by unrolling all my silk and soaking it for ten minutes in a container of warm water, to which I add a few drops of washing-up liquid.

Next, I take an absorbent (and above all lint-free) cloth in my left hand and pinch the end of my silk. With my right hand, I pull on the silk so that it makes full contact with the cloth. You can see from the color of the cloth that the silk, which looked clean, wasn't really clean.

If the silk were really dirty, this cleaning step would have to be repeated a second time.
After this step, the silk is clean and dry. All that remains is to apply a care product. All brands offer them. I haven't tested them all, but I assume they're more or less similar. I use JMC jm'z clean silk synthetic and Snowbee fly line slick, which I'm happy with. I know that some people use conditioner, others use plastic renovator products. When you look at the composition, it's scary. Even if the dose is very low, our waterways, which are already subjected to enough pollution, don't deserve this.
So I prefer to use products that have been specially created for this purpose and are free from toxic substances.

To apply them, I use a thin cloth or sometimes nothing at all. I put a drop of product on the tip of my index finger, then spread it a little with my thumb and run my silk between my fingers, refilling with product as needed.

During the operation, I place a newspaper underneath, which holds the treated line in large turns. I leave it like this for at least an hour before winding it back onto the reel.

That's all there is to it. Little effort for a lot of fishing benefits.
I repeat this operation about twice a season.
In addition to this thorough maintenance, I apply a little grease before almost every outing. Only on the last few meters of line and the body of my leader. It only takes a few seconds and ensures a few hours of happiness on the water.