Can i start by saying we are not builders/experts we are meerly showigng what we did. 

As you can see from our Tik Tok video there is lots of various advise out there in regards to floor insullation but this is just how we did ours. 

Firstly make sure that your subfloor is free from any debris, bots of mrotart on the floor etc so that you have a fairly level serface to lay the insulation flat on the floor. Once you have that done you can install the upstands all around the exterior walls of the house. For this we used 50mm insulation cut into strips. 
Then we laid 120mm floor insulation downstairs and 20mm upstairs. Lay as many of the large pieces of floor insulation on the floor then cut strips to size to pack in the floor, laying them as tightly as possible. We used Expansion foam around the pipes to ensure a seal. 

Some people reccomend taping all of the joints but Its not something we did. 
On top of this then we laid polythene, Underfloor heating pipes and screed.