Lay towel flat. Place kitty in center, so longest parts of towel are to the left and right of kitty. Fold one end over bum, then wrap the sides over, tucking the first side as tightly as possible around the cat and the other side over top to secure the first. The open end (where the head and front claws are) is then folded around the front, securing the paws/claws and leaving the head out for medicating.
If kitty is slightly more amenable, as in not fighting all that hard, but swatty, use same towel to make a bib around the neck/chest, with enough length to thwart swatting. Not tight enough to choke the kitty, but hold the towel and scruff at the same time so towel stays in place and so does kitty. At least in theory.
Here’s a video from Cornell on giving liquid meds.
Here’s one on giving pills or capsules.
You can practice with plain water, or try diluting the clavamox with water (once you have the proper dose in the syringe first) which makes more to give but might make it a little more palatable - you can even try low sodium broth. Or try a little bit in wet gooshy, yummy food and if that goes well, give the doses in food - this is only recommended if other methods fail, and if kitty will indeed finish all of the food or won’t get the proper dose.
If all of these fail, how much is the dose? Is it close to 1cc? If so, take it back and ask for the 62.5mg pills, pilling might go better.