How do I collimate my binoculars please because they have double vision.
It'll depend a bit on the root of the problem.
You can get double vision from properly aligned binoculars if the optics are too far apart or too close, relative to the spacing of your eyes. you'll need to bend them at the central hinge to make sure that both eyes are centered on the input lenses.
If you're still getting the migraine-inducing double image, they probably got dropped at some point. This can be a wide range of difficulty to fix. Here's what I've done in some past cases with flea market binocs that I bought cheap due to the skewed alignment.
For many binocs, the upper (eyeball end) and/or lower (field end) are screwed to the central body of the binocs, where all the folding optics are to widen the lenses from your eye spacing. If they have been dropped, (or disassembled and put back together sloppy), then they may be cockeyed at one of these joins. Look at each seam to the central body and see if any of them have a gap, especially one that is larger on one side than another. You can then apply firm un-screwing twist and see if it will unscrew. if it will, and before you unscrew it all the way, wash your hands and clear a clean surface to work on. Unscrew the segment, inspect the threads for crud that might have got in while cockeyed. Then visually square up the component and gently screw it back in. if it was mis-assembled, the threads may be a bit gummed, and you'll have to be patient to keep the optic square, and back off and start again if it tries to drag over to cockeyed threading. Once it's square, gently working the thread back into place by turning back and forth a half turn back, 3/4 forward, half back, 3/4 forward, can help the thread recuperate. Eventually, it should screw all the way in, firmly, and be square. No more double vision.
For some models, like my Nikon fixed pocket binocs, there's no zoom and just a focus knob. On my brothers pair, it got cockeyed. In that case, I gently unscrewed the locking rings for the glass on the eyeball side. One of them was loose and the lens was a bit cockeyed. In that case, I removed, it (wearing nitrile gloves), cleaned it with eyeglasses cleaning solution and a microfiber cloth, put it back in, screwed the ring in firmly,and it was good to go.
Bottom line, if you have some basic DIY skills, unscrewing the entry optics, or the optics sections, isn't too big a deal. Just be mindful not to let dust (or worse) get inside while you have things open. You can get a lot of fix just from that much.