Here are some pictures of a huge plush d20 I made. It was my first ever sewing project so if I could do it, anyone can. For an indication of size, see the third picture below.
This project is largely based off another project: https://www.instructables.com/id/Plush-Dice/
I did not record pictures of the process, but I will describe it here:
MATERIALS: Red fabric, white fabric, red thread, white thread, needle, stuffing (I got mine from two pillows), PVA glue.
1. Design - This project just uses lots (20) of triangles. Find the height, width and area with lots of maths and stuff and determine how much fabric you need.
2. Template - seeing as there are 20 identical triangles to be made, a paper template should be cut out to trace these. You will need 2 concentric triangles - the inside is the body and the outside 'border' is for sewing clearance. My triangles had 30cm sides.
3. Trace - Use chalk/pencil to put this template onto the fabric.
4. Cut - Cut around the outside of the triangles.
5. Numbers - Cut out your numbers. I made a template by printing out numbers on thin card and tracing them on the white fabric. Cutting it out was painful.
6. Glue - Glue the numbers on the centres of the triangles.
7. Sew - Sew the edges of the insides of the triangles together. This step is pretty much described in the instructable link above. For orientation and placement of the numbers, its easiest when you have a d20 to model off (which I didn't at the time). It is possible to look at pictures off the internet and work it out from there too (which I did).
8. Corners - The nearly finished die (with one edge incomplete to turn inside out later) should have the corners stitched by hand a few times, just to make sure holes don't appear in them and stuffing leaks out.
9. Stuff - the fun part. Use wolframalpha to work out the volume of the plush and stuff accordingly.
10. Finish - hand sew the last edge and you're done! :)
This is not an easy project, probably way to ambitious for my first sewing project, but it pays off well. My only tip would be to spend lots of time planning - orientation, placement, corners, inside-out-iness, amount of stuffing, size of triangles... these all should be sorted out before starting.