Apparently, it is possible to bonsai a horse chestnut, but I have never tried. I usually pick a long lived plant that grows slowly and has small leaves so it won't look disproportionate when downsizing.
You can start by restricting the roots. You will need to use a bonsai media which is not garden soil. I use cinder because it is readily available. Akadama is the usual media, but it is expensive and for training you just need a media that drains well. Most bonsai need to be outside and watered daily. If you water less or you live were the soil stays wet longer, you would use a material with more drainage. If you water less or your soil dries out very fast, then you could put more peatmoss or potting soil in the mix. You may have to cut some of the leaves in half if you prune the roots. It looks like the plant may have a potassium or magnesium deficiency by the color of the leaves. So, before repotting, try feeding the plant with a water soluble Miracle grow for Azaleas, Rhodedendrens, formerely Miracid. And see if the color perks up in about a month. In the meantime learn more about basic bonsai.
You will have to restrict the root and prune when it buds to make the leaves smaller.
https://www.bonsaiempire.com/basics/cultivation
horse chestnut bonsai 1
