I have boxes of those casters you linked to. So basically you just want to put regular wooden legs on it right? It is actually quite an easy task. What couch has those on there, just curious?

The Caster fits into an insert in the wood. First you must remove that insert/grommet. Then screw in one of these inserts using an allen wrench. Chose 5/16th or 1/4 according to the leg you pick out. Slightly oversize the hole...carefully. You can just wiggle a bit around if you want. https://www.amazon.com/Z-Threaded-Hex-Flush-Internal-Threads/dp/B002713X8G/ref=sr_1_4?keywords=5%2F16+insert&qid=1556762868&s=gateway&sr=8-4 When you screw these in do it in the same method of using a tap/die set. They are easy to bend. Do a back/forth motion. If it gets tight pull out entirely, enlarge hole. Grab a new one if you feel like you bent it. If your whole wasnt large enough its easy to bend those. If you screw up use one of those easy out screw stripped out removal tools. I buy the cheapy ones like this but other inserts don't have the issues of easily bending.

Your main bolt sizes in furniture are 5/16 & 1/4 inch ... and their metric counter parts (#6 or #8) . You will be using one of those. Probably American 5/16th when you buy legs. Just stain them some kind of black/brown.... that gos with anything.

If your hole is too large for some reason. Glue in a dowel with the holes diameter/depth(your starting over from scratch). Let dry. Then create the appropriate size hole with a drill starting in the center of the dowel.

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