Tthe mechanic for this is to get them both into one cell as a compound.
There is not really a tutorial for that mechanic (sorry about that) so here is a short explanation on how to get that:
You need a total mass of 202 (201 for Hg, 1 for H) in a single cell. You can get that by stacking a certain number of lighter materials on top of each other, these all need to be the same element. When you have that you can fuse them together and get the HG-H compound.
A example would be to use 202 H, stack them all together and then fuse once. This is obviously not a good solution since it requires tons of steps but I dont want to give away the optimal way ;)