oxygen bleach!

Find something that will hold the sheets with some room to spare, I love a rubbermaid tub for this.

Add 1/2 - 2/3 of the water you think you will need. It's easy to end up with too much water so go easy. Use the hottest water the fabric can handle, should be on the label if not look up the fabric composition and water temperature. For cotton sheets you can use the hottest water that will come out of your tap.

Then add the Oxi powder, I started by reading the instructions but now I go by vibes and the colour of the water. For sheets I would probably use 2 scoops, maybe 3 depending on the size of the tub.

Add the sheets and give it a bit of a stir. I have a sturdy silicone mixing/serving spoon I use for this.

Now figure out if you need to add more water/oxi and add it. You're looking for a sheet stew vibe, enough water for there to be complete coverage and some room for movement. Extra water dilutes the solution and is heavy but you do want enough to submerge and agitate a bit.

Then let it soak, stirring occasionally, at least until the water cools. I would leave sheets all day or overnight but when I do clothes in a bucket I will usually only leave a few hours.

Drain and wring them out best you can and wash as normal. I usually add an extra rinse just because oxi can make things a bit stiff if not washed all the way out. You can repeat this process if needed.

Hope that's helpful and you end up with clean stain free sheets!