Simply take the terminator storm bolter, a melta gun or a plasma pistol and start melding the bitz together. You will also need a barrell from a spare bolter or bolt pistol and a little bit of Green Stuff modelling putty.
To make a combi-melta, simply chop the barrels off the storm bolter (leaving the stubbing bit of plastic underneath intact. Then cut whatever detail is on the side of the storm boler off, leaving a smooth, clean surface.
Take the melta gun, chop the barrel off, give it a trim where needed and glue it where the right barrel on the storm bolter was. Then carefully cut the fuel tank and the two wires coming out of if, off the side of the melta gun. I tend to gradually force my blade along the side of the melta gun, under the fuel tank and out the other side, then tidy up afterwards. It doesn't have to be a perfect cut. Polestyrene cement glue should melt the plastic to fill any small gaps.
Glue the melta gun fuel tank to the side of the storm bolter than you made nice and smooth.
Finally, cut the barrel (and sight) from a bolt gun or boltpistol, tidy it up and glue it in place where the left barrell on the storm bolter would have been.
And you've go yourself a quick and easy combi-weapon!
The combi-plasma is a little more tricky. For this you need a plasma pistol cut from a space marine's hand. I find that it's best to cut the hand off at the wrist, then line your knife blade up to where the hand lies flat against the underside of the gun, then cut straight through and tidy up where needed to create a smooth, flat surface.
Just like before, to make this combi-weapon you need to chop the barrels off the storm bolter AND cut off the stubbing bit of plastic underneath to make room for the plasma pistol barrel.
Now comes the difficult part.
Because the plasma pistol has details that can't be chopped off and stuck on to the storm bolter, we need to trim down the plasma pistol, cut a section out of the storm bolter and place the trimmed plasma pistol inside, glue it into place and then fill the gap with green stuff and shape it to resemble the stock of the gun.
I used a knife to cut straight down the centre of the gun stock (be careful to leave the magazine intact!). I then cut in, from the side, in line with the point where the magazine meets the stock of the gun. I then cut across from the back of the stock which meets the part where the shell canisters are dispensed.
I was eventually able to cut a rectangle of plastic out of the storm bolter stock for the plasma pistol to fit into...after a bit of trimming.
Take a knife and smooth the left side of a plasma pistol until it's nice and flat, then using polestyrene cement glue, fix it into place. This glue is a godsend because it will melt the plastic together, making this difficult conversion look a whole lot better!
Leave this to dry for at least 3 hours. It's incredibly fragile until its set, so you need to leave it well alone before you do any more work.
You'll notice that there's a gap where the plasma pistol grip would normally go, because you've carved this out of the storm bolter stock. Simply fill it with Green Stuff modelling putter and shape it into a square/rectangle to mimic the overall shape of the storm bolter and help blend the plasma pistol parts into the stock of the combi-weapon.
Finally, take a spare bolt gun or bolt pistol, carefully cut the barrel and sight off in one piece and glue them where the left barrel on the storm bolter should be.
Hopefully that's been a big help for anyone wanting to give their Terminators combi-weapons. I haven't done a combi-flamer conversion, because it's more cost effective to take one of the other weapons or a Heavy Flamer.