Our house is a 1924 detached property with suspended floors and cavity walls. The walls were retrofitted (badly) with insulation some time in the 80s.
The front hallway and one front room of this property has access to a crawlspace. It is not very high but has been used by (presumably small) plumbers and electricians. If you are lucky enough to have such a crawlspace, then underfloor insulation is relatively easy to fit. We used two materials each with a different method: Multifoil and Blanket.
Insulation installed via crawlspace
Multifoil
Multifoils are primarily used for loft insulation but also works well underfloors. It has exceptionally good insulation properties. The one we used, Superquilt, is a relatively thin layer of foil and other materials and when used underneath a floor it is fixed across the bottom of the joists so as to create a relatively airtight gap between the joists. You also need to make sure the floorboards above to not have huge gaps between them.
In practice this wasn’t as easy as I thought. Stapling while in a cramped space in an awkward position is not easy. My only power stapler was a light duty one with staples only just big enough to hold the quilt in place. Mostly I used my heavy duty hand stapler but this was much more difficult. If I was to do this again I would invest in a heavy duty power stapler.
Although you do not need 100% airtight gap you do need to ensure that it is as relatively airtight. To create the seal it can be stapled to battens at the side walls and/or taped to the walls, however this is not so easy unless the walls are quite regular. The crawlspace under our hallway had central supporting piers, which would have made this quite awkward so we opted for a different method (see below). The main difficulty we experienced was getting the best seal when radiator pipework is in the way (see image)
Blanket
Blanket is thicker than multi foil and in our case we used 100mm as the joists are about that size. With larger joists a thicker or double layer would have provided better insulation, however the 100mm was enough to get it included in the EPC rating. This ended up being a lot easier than the multi foil because you are not relying on any kind of air seal, just the thickness of the material. So we used this on areas that were more irregular.
You start off by wedging the insulation between the joists. Although it can be a nicely snug fit between the joists, it does need to be held in place as it could fall out later. You can do this by stapling chicken wire or similar as a “cage.” Breathable roofing felt membrane would also work.
We could have used fibreglass or rock wool, but it is horrible to work with. It can be hazardous to breathe around and can cause itching. We chose instead a modern type of blanket that has the advantage that it is made from recycled plastic fibre and is much nicer to work with. The material we used is actually call Non-Itch™
Insulation installed via board lifting
We also insulated our lounge floor, but there was no viable crawl space as there was only about 500mm below the suspended floor boards. We could have done this with multi foil quilt or withy Celotex board, but either of those methods would have involved lifting all the floorboards, so we decided on the easier DIY option to slide the non-itch blanket underneath by removing every fifth or sixth boards. In practice it made most sense to lift the boards that were easiest so it wasn’t always the fifth or sixth. Some stretched all the way between both skirtings, but others had already been cut by plumbers so these are the ones we lifted.
Next step is to cut the insulation into short lengths to feed under and (importantly) find a way to stop them falling out. Again, you can staple chicken wire under the joists by reaching under, but we chose a different method, purely because I don’t like cutting chicken wire. 
(Oops, the stapling could have been better, will go back and fix that!)
We also had some battening lying around so we screwed battens under the joists and stapled three “rails” of strapping. This is the stuff they used for packaging. It’s easy to screw the battens diagonally from above, much easier than attempting to screw them in below the joists.
With the strapping rails in place it is very easy to slide the insulation along between the joists:
Other materials.
For both of the above we could have used a solid board such as Celotex or Kingspan, but this requires careful cutting and is more expensive. As well as the recycled plastic batting, natural materials such as sheep wool are also available.
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