Your fork should have a spec which either specifies a maximum number of spacers between the upper bearing and the bottom of the stem or the total height of the steer tube between the upper bearing and the top of the stem. For a carbon fiber steer tube, it's usually those specs are 35-45mm and 75-85mm respectively. Just contact Lynskey if the fork didn't come with any specs or instructions.

That's where you should look for info, not here.

My experience is strictly with carbon fiber steer tubes.

You use a new, sharp, fine tooth hack saw. They make saw guides for this purpose to ensure the cut is square, and they're not expensive, but I've always just carefully done it free hand. I wrap a band of masking tape around the tube, mark a line around the entire tube for the cut line, and carefully cut keeping on that line. It comes out nearly perfectly square, and I make fine modifications of the cut with a fine file - if necessary - to square it up. But it doesn't have to be gnat's a$$ perfect. I've also tried a miter box as well as the "two hose clamp" method, but didn't think it came out any better than carefully cutting along a line as described. The saw guide is probably more fool proof. But I never bought one and every time I cut a steer tube, I think it's the last so I don't bother getting one. That was about 5 steer tubes ago.

When I'm done with the cut, I lightly and carefully file or sand the outer and inner edges of the cut to lightly dull the sharp edges and reduce the chance of splinters. Look on Youtube for instructions