Although roll top baths don't require a bath panel, most standard modern baths do need some kind of panel to cover up the underside of the bath and hide the plumbing.  It makes sense to make sure whatever panel you fit is removable, so you can access the plumbing as necessary.  Moulded wooden panels are commonly used, but bath panels can also be made from MDF, which can then be tiled. 

First of all, you'll need to construct a frame to attach your panel to, unless you're using an acrylic panel, in which case the framework will be inbuilt.  If you're using a wooden bath panel, wooden batten is great for making your frame.  Take a piece of 2in x 1in batten and lay it on the floor parallel to the rim of the bath.  Then place a small block of wood, which is the same depth as your panel, next to the batten and put your spirit level vertically on the outside of that so the top touches the top of the bath.  Then move the block and the spirit level accordingly until the batten is completely in line.  Use a pencil to draw along the batten and mark on the floor where it needs to go.

If you need an end panel as well as a side panel, you'll need to repeat this step to determine the position of the end batten.  Measure the space between where the pencil lines intersect and the walls and cut your battens to the correct length. 

The next step is to fix the longer batten to the floor.  Then measure out a length of batten to go from the underside rim of the bath to the top of the base batten.  Fix this at right angles to the top of the shorter base batten, which you then need to fix to the floor.  Make sure the batten you've attached to it runs up to the corner of the bath rim so it adds extra support.  The last stage of building the framework is to fix battens to the walls at each end of the bath, which can be used to fix your panels to.

Bath panels come in standard sizes, which you will need to adapt to fit the space in your bathroom.  You will probably need to scribe your panel so it sits flush with the wall and the skirting board.  To do this, use a pencil to mark out where you need to cut in order for the panel to fit around the skirting board.  Carefully, using a fine cutting blade, use a panel saw or jigsaw to cut out this piece.  Then screw your panels in place, screwing directly into the batten frame.  Once they're in place, drill pilot holes through the end panel where it joins the main panel and use mirror screws to secure them together.  Alternatively, you could use magnetic catches in place of the screw fixings.