I wanted to make a replica of the One Ring from Lord of the Rings but I also needed it to be quite visual. I decided to go for a necklace like approach (as in the films) but with quite a lot larger ring (so it could be seen from a distance.

Step 1: Gather Materials

Materials used:

String (about 3 metres)
EVA foam (about 15cm x 15cm)
Safety pin (explained later)

Tools used:

All purpose primer (I used grey to have a better look)
Gloss gold spray paint
Clamp
Black, fine-tipped felt pen
(optional) Heat source - hairdryer, heat gun or candle

Step 2: Making the Necklace

Some points to note for this step:

The necklace is made from a simple 5-way plait as described below.
The start is a bit tricky but you can get into the flow of things after making a bit of the plait.
The strands are numbered from the left to the right as 1, 2, 3, 4 and 5. The strands will move position so what was strand 1 might be strand 2 in the next step. For each step, the leftmost strand is referred to as strand 1 all the way across to strand 5 on the right.
This stage will take quite some time (it easily took the longest time for me). I would advise doing some of the other steps when you get bored of plaiting. Ideally, you could alternate spray painting coats onto the ring with plaiting another 15 cm so that you don't get tired of this stage. I think it will depend on your boredom threshold.
The ends will get tangled quite easily so separate the working ends as often as you need to. I tended to re-clamp the plait and untangle at the same time.
You want to keep a fairly constant tension on the strings as you do this (it doesn't need to be much). If you don't the pattern of the plait will bunch up in places and stretch out in others.
Cut the string into five equal lengths. (I think I went for 60 cm per strand - see improvements at the end.)

Process:

The way I made this was to tie a simple overhand knot with the five strands of string.
Clamp the knot at the edge of a table.
Take strand 3 over strand 2. Take strand 5 over strand 4. Keep these about the same height up the plait.
Slightly lower than step 4, take strand 1 over strand 2 and take strand 3 over strand 4.
Lower again than step 5, take strand 3 over strand 2.
And lower again, take strand 1 over strand 2.
You should be able to see a pattern forming here. From a similar height to step 6, take strand 5 over strand 4, under strand 3, over strand 2 and under strand 1. This strand (strand 5) should weave diagonally down and to the left.
There will now be a new strand 5. Use this new strand to repeat step 8 but progressing down a little bit each time.
Repeat until most of the length is used up. It is advantageous to re-clamp the complete plait after 5-10 cm so that it doesn't unravel.
Once most of the string has been used up, tie the strands into a knot so that the plait doesn't unravel.

Step 3: The Ring

This was quite simple to make. Step 6 is optional and involves heat or flames - it's not essential but it gives a better finish.

Use a large shape (about 15 cm in diameter) and trace around the shape onto the EVA foam.
Find a smaller shape (about 10 cm in diameter) and trace around that within the circle from step 1.
Carefully cut around the the outside ring and then cut out the center circle. You should be left with a ring which is about 5 cm wide.
Trim the edges to make it circular, as necessary.
I slightly rounded the edges to have a better shape and slightly conceal the errors.
(optional) Use a heat source to close the foam cells so that it doesn't absorb the paint quite so easily. (EVA foam is known to do this. The foam should turn slightly glossy which is the surface finish you're looking for. If you're using a flame or a high powered heat source, be very careful that you don't burn yourself or set the foam on fire. I use gardening gloves to insulate me from the heat.

(I forgot to take a picture of the unpainted ring but it's a fairly obvious shape.)

Step 4: Painting

The painting was pretty simple. Remember that several thin coats are better than one thicker coat - thin coats will create better coverage and not pool in areas. It will take longer but it will look better.

Ring
Give the ring 1 coat of primer or two coats if you didn't heat treat the ring in part 6 of "Step 3: The Ring". It should be the colour of the primer (grey for me) with none of the original colour showing through (green for me). Allow enough time for the primer to dry completely, however many layers you use. Make sure you cover all of the foam with primer - this might mean you have to turn the ring over while the paint is damp. Alternatively, spray one side and then spray the other side when the first side is dry.
Using the glossy gold paint, apply several thin layers to build up a glossy finish. Allow suitable drying time between each coat. I think I used three coats in the end. You could use a clear lacquer to seal the paint but I didn't.

Necklace

For this, I light sprayed the string with the same gold paint but with only one coat. I deemed this enough to look like a well-worn, gold necklace. Any more coats might have made the string less flexible and less comfortable to wear.

Step 5: Inscription

I found the black and white image on Google and used that as a reference material. Using that, I wrote the Elvish letters around the painted ring. Once again, you could probably use a clear lacquer to seal the inscription but I didn't.

You could trace it or just write it out (I did the latter) but remember to take it slow and steady. Keeping the style will fool most people at a quick glance, even if there is a mistake or two. Taking your time should give an accurate result.

Step 6: Putting It All Together

Thread the string through the ring in whichever way you want. As I've done it, it will twist the ring a bit. If you use a lark's head knot, it'll lie flatter but will use up more of the length of the necklace.

I used a safety pin to close the necklace but you could just tie the ends together if you have enough slack for the person who will be wearing it. I quite like the safety pin approach since it's unlikely to be seen (the safety pin will be behind the neck of the wearer) and allows for quick and easy adjustments, if needed.

Step 7: Improvements

I have thought of several improvements that I could use if I were to do this project again. In no particular order:

I should have allowed more string per strand. By plaiting it, you use up a fair bit of the length to make the pattern. In hindsight, I'd probably use 90 cm per strand and then possibly trim it down rather than using a safety pin to make up the extra length of necklace. I think a suitable knot would look nicer but the back of the necklace is fairly well hidden anyway.
Sealing the ring with lacquer might prevent the cracks that have appeared on the ring. I think it adds to the effect a bit.
The necklace could be substituted for an actual gold necklace but I don't have access to one of those and this is meant to be a prop. I would be upset if a borrowed, real gold necklace were to be broken while wearing this (considerably cheaper) prop.
A different material could be used for the ring. Wood is easy to work with and can be painted easily but might add a bit to the weight of it. Some sort of plastic could be a robust alternative. Using brass (or something similar) would look very nice but would add a lot to the weight and overall cost. The inscription would likely need to be engraved rather than written. Maybe a metal version would be a possibility for the future.

Introduction: The One Ring Prop