The easy way to get one is to take either a drink bottle or a detergent bottle with a 'pop-up' lid and use that, as is. You can use these to squirt upwards or downwards, but it's messy, and if you want a gentle and controllable water flow, you need a proper wash bottle.
The tubing has to reach the bottom of the bottle inside, and come part of the way down on the outside, as you can see from the third picture, so work out the length you need for your bottle. Choose a drill bit that makes a tight hole for your tubing: I use a 13/64" (5 mm) drill bit, but you need to test this for your tubing. Use an old cap and drill several test holes to choose the right size that gives a tight fit.
|The wash bottle looks like this when it is assembled.|
Then take the cap off the bottle, drill a single hole in it and feed the tubing through the hole until there is enough to reach the bottom of the bottle.
After you have fitted the cap onto the bottle, wind a piece of wire around the tubing so the tubing will take and hold whatever shape you give it. Then cover the sharp ends of the wire with gaffer tape, add water to the bottle, and you are ready to go. You can also use a small amount of tape around the tubing to make a tighter fit where it goes through the cap: this trick also works for pooters.
|The wash bottle, ready to use.|
Still with me? Good.
You can adapt the same design to make a water sampler bottle. I will cover this briefly, because you can experiment with this yourself.
You don't want the water coming out when you squeeze, so if you are holding the bottle right-way-up there should only be a short length of tube inside the bottle. You can also use a long tube attached to a stick to draw up samples from deeper water, but remember to squeeze the bottle before you put the tube in the water, so bubbles don't chase off the wildlife or stir up the sediment too much.
With a bit of thought, you can probably use the same design for a simple one-tube pooter for catching ants. You need a fairly tight seal where the tube goes through the bottle cap, and I sometimes use epoxy resin glue to seal the tube in place. I advise you to get adult help to experiment with epoxy resin. Work outside, don't breathe the fumes, and try not to get it on your skin. Epoxy resin isn't that dangerous, but play safe!