Well, nice that no one's actually offered you any advice given that you've made it clear you're going to IV it....8(
Obviously it goes without saying that by starting to IV stims you're opening a whole other can of worms with many problems and that it's best not to do that, but if you insist:
1. Usual sterile (as possible) injection practice.
2. Do not add heat! It will make it congeal and it will either be impossible to draw up, or even worse you will manage to draw some up and inject it and it will really do your veins a mischief. So...
3. Start small, like 0.1. UK rocks are usually gash but if you chuck loads in the spoon and do a decent job of cooking it up it's probably going to blow your socks off. IV is NOT a shit route of admin if you cook up well. Smoke a stone the size of the one you have in one go and you will probably end up breathing half of the cocaine out unless you have some mega lung power. Cook it up properly and every last bit of that stuff will be going to your brain. So be careful! You can always add more you can't take it away when you're spazzing out on the floor halfway through a seizure.
4. If you want a speedball cook your gear as normal but with extra water and let it cool down in the spoon. I would suggest you just try it alone first though, in which case just start with some tepid water. From here add extra (or some if you are starting by itself) citric. Either crush the rock and chuck it in, or just chuck it in the spoon and crush it in the water with the back of a pin. Now let it sit for a few minutes, stirring occasionally. NO HEAT! As UK rocks are shit, you will see that the majority of it won't dissolve. This matters not, the actual cocaine in it will dissolve easy, anything left is shit you don't want. Draw up as normal through a filter.
5. Inject. Personally I always inject a tiny bit first and wait a second or two when injecting cocaine as I've accidentally hit an artery injecting it before and didn't realise because of the numbing effect meaning I dumped the whole pin in to the artery. Believe you really don't want this to happen! That's just a personal preference out of paranoia of the same thing happening again and isn't usual practice though reallly.