Having boundaries is the first step - recognising what your boundaries are and experiencing the feelings when someone violates them.
Holding boundaries is much harder. You've built up patterns over the past three decades where his desires are prioritised over your boundaries, and there will be lots of reasons given why that is reasonable, to the point that you are really uncertain whether you are being reasonable, or possibly even what you want.
To assert your boundary, you need to express it clearly, and there needs to be a consequence if your boundary is ignored. Actually holding boundaries and expecting them to be respected is the first one.
This is so, so hard, because you are scared of the consequences. It is entirely reasonable to find this frightening.
Have a look at how your husband responds to other people's boundaries - people he respects. I bet he respects their boundaries - I bet he anticipates some people's boundaries and doesn't go near them. So he can do it, he's just choosing not to.
You can learn to hold boundaries. I've been through this in relationships with various people. Once you start doing it, it feels liberating and you realise that what you want is important. You need people to support you - women's aid, family members, friends - when you are wavering they can remind you why it matters. An outside eye to recognise patterns of behaviour, and encourage you to persist (because it's so tempting to just give in 'on this occasion') makes a massive difference.
I really hope you manage to do this. He has no right to touch you when you don't want to be touched or denand that you walk around naked.