This instructable owes a serious debt to xinistrom's "Plastic bottles to nunchucks (and more)": https://www.instructables.com/id/plastic-bottles-to-nunchucks-and-more/

You will need:
1 empty 2 litre plastic bottle - for the 'blade'
1 empty plastic bottle with a larger neck - for the 'hilt'
Decent scissors
Clear Sellotape (nice and wide if you can get it)
Craft /stanley/xacto knife
Silver Tape (Duct Tape)
Sandpaper
Superglue

Optional extras:
A4 Sheet of coloured paper
Or the multicoloured LED from inside a toy which flashes when squeezed or whacked.

Step 1: Plastic Bottle Lightsaber - First Cuts

Wash the bottles and remove the labels.

Follow the cuts marked on Picture 1.
Cut the top and bottom off to leave a large plastic tube.
Vertically cut that plastic tube into two, there is usually at least one moldline you can follow.
Then cut the very top of the bottle off, the bit that the cap screws onto.
The rest of the top and bottom can then be binned/recycled.

You should be left with the cap screw and two curved sheets of plastic as per Picture 2.

Fold both sheets flat(ish), place one overlapping the other and stick a bit of tape to secure them together. Roll them up into the tube shape they'll naturally try to form while being careful not to disturb the tape you stuck down earlier (See Picture 3).

Step 2: Plastic Bottle Lightsaber - Choose Your Colour

Choice time, either you can make a clear blade or a coloured blade. I'm making a clear blade but here's how to add colour.

OPTIONAL: If you want a coloured blade you could take an A4 sheet of coloured paper, cut it into two (lengthways), overlap and tape the pieces just like we did for the plastic tubes (See Picture 4) and then roll that inside the tube (See Picture 5).
Of course you could just look out for a coloured plastic bottle and use that but I don't see too many red or purple bottles, lots of green though.

It's time to close this blade up. Roll the tube tight, tight enough so you can slip the screw end over it snugly (See Picture 6), move the screw end close to the middle and securely sellotape the middle down, then move the screw end to each end and secure those with tape, and then I'd recommend taping 1/4 of the way from the end of the blade as well so it's secured at 5 points.

I'd trim or sand any sharp or wonky edges off the end of the blade now, we will secure it properly later.

Step 3: Plastic Bottle Lightsaber - Making the Hilt (part 1)

Slip the screw end off and tightly wipe sellotape in a corkscrew fashion over the entire blade. Start at the top and make sure there's an overlap of at least 1cm so you can fold that back into the blade and secure any sharp edges from the plastic. Make sure there's an overlap at the other end and push that back in as well (See Picture 7). Slip the screw end back on near the end of the blade and make sue the screw section is pointing to the shorter end of the blade. This is going to be our Blade Emitter.

Take your 2nd bottle with the larger cap, this is going to form the hilt so you might like to pick a bottle with some kind of pattern.
Make cuts similar to the first bottle. You might want to only make one cut 'up the tube' so you have more material to work with.
Ensure you keep the screw end and also the bottle cap this time. (See Picture 8)

OPTIONAL: Now I kept the blade clear as I was going to try adding in a multicoloured LED from inside a broken toy. The LED flashes different colours when dropped or hit so I'm hoping it'll flash when the blade is struck. this is a small sphere so I've taped that to be bottom of the blade.

Roll the 2nd plastic plastic sheet into a tube, slip the screw end over it and put the short end of the blade into it as shown in Picture 9.

Step 4: Plastic Bottle Lightsaber - Making the Hilt (part 2)

Roughly measure the overlap between the blade and the hilt, make sure it's about 2.5cm, move the blade's screw end so it's touching the top of the hilt. Take the blade out of the hilt.

Trim excess plastic off the hilt ends and then put 3 strips of sellotape on to the middle and ends to secure it like you did back on Picture 6.
Take the screw end of the hilt and screw the cap back onto the screw end.
Put the blade back into the hilt. now carefully push the blade's screw end up and secure the the hilt to the blade with tape. Start covering the hilt in sellotape in a corkscrew fashion like before. Allow some overlap at the end to push back into the hilt.
Push the blade's screw end back down over the securing sellotape and slip the other screw end with it's cap onto the end of the hilt like in Picture 10.

Stop and admire your work, it's taking shape! (Picture 11)

I'd slip the hilt's screw end off and drop a bit of superglue inside it to help it stick to the hilt. (Picture 12)

Step 5: Plastic Bottle Lightsaber - Personalising the Hilt

Time to get the silver tape out!
Pop the cap off the hilt end and carefully wrap everything from the blade emitter to the end of the hilt in silver tape, be careful to ensure the cap will go back on over the tape. (See Picture 13)

Admire! (Picture 14) Then plan. The hilt is personal to each jedi, no two hilts should really be the same. Plan your personalisation and decorations, buttons, readouts, sticky out bits, what ever you fancy or can source.

Then put them into place (Picture 15), as this was a test run I've kept is simple, painting the bottle cap black, taking a sharpie to the hilt and bring out some out the details that was already there.

And that's it we're done! You are now one step closer to Jedi Knight status my Padawans.

ADDITIONAL: Well the flashing LED didn't work as expected when you hit the blade, it just wasn't powerful enough to register but it worked nicely when you bashed the hilt onto a table or floor. My 3 year old adored it and eventually lost it in the park since he didn't want to let it go. So I'm gearing up to make 3 new ones, one will be using a green bottle, another will have coloured paper in the blade and I'll try and source another LED for the third one.