This is where you discover how well different comfort layer materials and thicknesses can give you the pressure relief you need in a mattress. You will have ... from the other guidelines and from your own step 1 testing ... some general ideas in the types and thickness of comfort layers you will need. Now it's time to start testing these general preferences out. At this stage we are looking strictly for pressure relief and not in any way deciding on an overall mattress so once again ... as in step one, any mattress in the store regardless of price can be your "playground". This step involves lying on each mattress for longer and is where you will find out more about the qualities of the different materials and methods that provide comfort and relieve pressure ... and how well each one works for you. Trust the general patterns and preferences that came from your previous reading and the research in step 1 and try to choose mattresses for testing that have the thickness and softness that is "in the general area" of your preferences so far. You will also need a comfortable and appropriate pillow for this step as pressure issues (in the shoulder and hip areas especially) and feelings of how comfortable a mattress is for you can be greatly influenced by different thicknesses and types of pillows.
You want to specifically lie on a few mattresses that have the thickness and type of material you are interested in so far. If you have several possible preferences in materials, thickness, or softness, then lie on mattresses that include these preferences in the comfort layers on or very near the top. Don't forget, you are testing for comfort layers only and not for the "mattress as a whole". How well a mattress supports you is not important in this step and neither is the price of the mattresses you are using for the testing. Avoid trying to rate the "mattress as a whole" as this may well lead to poor comfort choices.
Using the previous guidelines and your testing experiences, lie on a few mattresses that seem like "reasonable choices" but this time for a little longer than you did in step 1. What you are looking for is their ability to conform to your body, relieve pressure at specific points of your body, and provide overall comfort (distribute pressure evenly and just plain feel comfortable). Spend some time on each of them in all your sleeping positions and pay particular attention to how comfortable they feel when you are still, while making small movements, when you are changing positions, and whether you can notice any pressure points in any of your sleeping positions or movements. You can "bounce" a bit, especially on your side, to see if you can feel a "hard" layer underneath your hips or shoulders or if it is still comfortable and "giving". You should not be hitting "rocks" or "boards" if you move or bounce a bit in any of your positions or when you change positions. If memory foam is one of your possibilities, make sure you lie on it for long enough for it to warm up to your sleeping positions. If you are looking at mattresses with more passive materials in the top layers (wool, cotton, horsehair, and others) make sure you test the effects of different types and strengths of springs under these layers because the differences in springs in these types of mattress materials will largely determine how conforming or pressure relieving the mattress is. If you do feel pressure in any of your choices or positions, use different thicknesses of pillow to see if that is the problem or if it is the mattress itself. Test slightly thicker comfort layers if it feels like you are going through the top few inches and hitting a harder layer underneath or slightly thinner layers if the comfort layers just feel like you are sinking in too much. If the salesperson is not helpful with telling you exactly the type and thickness of the comfort layers, then just ask them to leave you alone while you do your testing. All this information will be available online after you return home.
Try a few "combinations of materials" in an appropriate thickness to see how they feel. More than anything, try not to be drawn in to being too technical and trust your instincts as to which mattresses seem to provide the best pressure relief.
Also be aware that each category of material has a range of densities and softness/firmness that can greatly alter and/or change its ability to conform to your body, relieve pressure, and create a personal sense of comfort so try to avoid generalities about materials and work towards finding out the thickness and softness of the comfort layers that works best for you. By the end of this step you should be able to say "I tend to prefer latex, memory foam, natural stuffings on springs, or microcoils", on the "thicker/thinner and softer/firmer side and should be able to describe the main differences in feel, ability to conform, and comfort between different materials when they are the primary ingredients in the comfort layer of a mattress.
Ask lots of specific questions when you need to (what is this material exactly and what is the thickness, density, and ILD of the material are good examples). Get specific answers and if the person you are talking to can't or won't give them to you, be very very careful about trusting their opinions and advice on anything else. Take notes if you need to about pressure relief and comfort (none about price or support at this stage), and when you have narrowed down your choices to a few mattresses that seem to provide the best pressure relief, it's time to quit and take stock of where you are. When you get home and take a look at the makeup of the mattresses that you preferred for pressure relief, you will also see some patterns here. If you need help with this the forum is always available for your questions. Once you are clear on the type of materials and thickness you need for pressure relief it is time to turn your attention to support issues. We are now 2/3 of the way through your "field testing".