If you are looking to preserve the web itself then this can be trickier and also has the issue that you may be bringing other unwanted elements into your spell. Some witches dislike crafting with artificial ingredients but it would be near impossible to achieve this naturally (or at least it has been in my experience – I’d be interested to hear from anyone who has perfected a 100% natural method of preserving a web).
You can try fixing a web in place with hairspray. This is tricky to pull off. The best way to do it is to build the hairspray up in layers. If you spray straight at the web from a close distance it will break the web. Spray from the furthest difference (from both sides), let it dry and then keep repeating getting closer to the web each time.
You can also try laminating the web. Get some clear sticky backed plastic and peel off the sticky side. Hold the sticky side level with the web and gently move it forward until the web is captured onto the stick surface. Cover with the other side of the plastic (or laminating pouch to preserve). You could also do something similar with sticky backed plastic and a coat of varnish over the top.
What ever method you use, remember it is impolite to deprive a spider of its home so try only to use an abandoned web.