How do you thicken your blood before surgery?

In general, most people would not want to thicken their blood before surgery, since it would present a larger risk of blood clots.

However, assuming you are an individual who actually could benefit from improving the clotting quality of their blood before surgery... there isn't a huge amount which can be done without the aid of pharmaceuticals. There is an obvious (but silly) way to thicken your blood, and that's by being dehydrated. Clearly this is not a healthy thing to do, and if you were to turn up to surgery dehydrated you would probably be given IV fluids anyway - it's a pointless route to go down.

There is sufficient evidence to say that calcium and vitamin k play crucial roles during clotting - however unless you're actually deficient in these substances there is very little benefit (and potentially some harm) in taking extra in the form of supplements.

I think, in this instance, your doctor and surgeon are probably fully aware of your clotting issues and will have taken them into account when constructing a plan of surgery for you (e.g keeping blood products on hand, using tranexamic acid etc...). However if you are in doubt about this, speak to them. It's really their responsibility to ensure that your clotting function is safe for surgery, rather than your responsibility - it's not something that can be easily altered.