You can practice on scrap pieces of acrylic , lexan, polycarbonate sheet. Any shop cutting plastic would have scrap you could probably take for free.
Scratch a side gently with sandpaper, attach to a heatproof surface (cork, cardboard, garden slab) with double sided tape. Get cooking.
As said above, keep the flame moving but you need to get the surface temp of the plastic up whilst not heating the gasses in the plastic enough to cause bubbling. Its better to heat and cool and reheat a few times then it is to keep the heat on and end up with blisters or a hole.
Also, some plastics are porous and or absorbent meaning they can hold a moisture content similar to what is in the air, if one of these plastics is heated it will blister really easily if the moisture content is high so it’s advisable to put ramps in a warm place (airing cupboard) for a little while before to dry out.