ok, in a nut shell, you fit sheets
to a v berth mattress by making a pocket at the foot of the bed
to hold the sheet down.
The longer explanation;
You need a sewing machine
that can straight stitch, a pair of scissors, a pencil and some pins.
1 Start with a bare mattress.
2 Put the sheet on the mattress so that the wide end of the triangle (aft end) has the sheet stretched smooth and the fitted sheet corners well over the mattress corners.
3 Now smooth the bottom sheet out to the end of the berth, as far as it will go. Smooth it out to each side so that it's as flat as you can make it. neatness counts and will result in a better finished product. Mark the edge of the sheet along the sides and cut the 2 extra corners off.
4 You will be left with an odd triangular sheet with a corner at one point.
5 Fit the corner over the foot of the mattress and smooth up to cover the bottom edge of the top piece. Mark across and down the side on the bottom piece and trim.
6 put both pieces on the mattress and smooth them out then pin them together as best you can on the mattress without
actually pinning them to the mattress (tricky). Remove, mark along the pinned line and match the sides face together and seam along the edges along the pinned line.
The top sheet is easier. Fold it in half and stitch up a few feet along the side starting from the bottom edge of the sheet. Open it up and slide the pocket you have made over the foot of the mattress and the top sheet will stay tucked in and not create a lot of bulk under the foot of the mattress. This works with blankets as well, although for bulky ones trim the blanket and add sheeting to make the pocket.