If you can't get new slings professionally replaced on your climbing cams, here are several ways you may, or may NOT want to DIY.
Sewing a sling yourself: DO NOT DO THIS!
Girth hitch: NOT recommended - Extra bad for cable, low results
Basket hitch: Ok if you clip both strands - http://www.kakibusok.plus.com/Equipment/ReslingCams/Resling.htm 
Bowline on a bight: Ok but knot weakens slings
Beer Knot: Ok - Bulky and remember water knots are sketchy https://youtu.be/mXe-8GmS08k
7mm Nylon cord: Meh - bulky and isn't stronger than a sling

Buy slings at https://hownot2.com/collections/climbing-slings

****Professional Options****
BRANDS
Black Diamond - https://www.blackdiamondequipment.com/en_US/content/cam-repair/
Metolius - https://www.metoliusclimbing.com/pdf/cam_repair_form.pdf
DMM - https://dmmwales.com/pages/warranty-returns-servicing-repairs
Totem - https://www.totemmt.com/wp-content/uploads/2020/04/TotemCam_repair_form.pdf

3RD PARTY
https://www.runoutcustoms.com/cam-repair--resling.html
https://www.mtntools.com/cat/rclimb/cams/mt_camresling.htm
https://www.yatesgear.com/index.php?route=product/product&path=223_405&product_id=1029 

A Polish climbing shop reslings cams - https://www.lhotse.pl/uslugiszwalnicze/

Want new cams? https://hownot2.com/collections/cams-1

Dangerous Tat episode https://youtu.be/IpJtTRmg7Og
Angel Cam episode https://youtu.be/wLDK6ZoH87Y
Walter's Seibert's death knot video https://youtu.be/mXe-8GmS08k
Water vs Beer knot episode https://youtu.be/sOCjyVUBkGY
Home made cam episode https://youtu.be/0AUWepe1UdI

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00:00 Reslinging Cams
01:13 Different cam slings
03:06 When should you replace slings
04:16 Sewing
05:08 Basket and Girth Hitch
09:58 Chris's death products "tip"
11:46 Bowline on a bight
15:24 Quicklink
16:49 Accessory Cord
18:06 Webbing - water knot
19:22 Professional options
22:41 European law
23:48 Totem Trick
25:26 Or it's free