Usually you take it IM as are the directions. HCG does not need to be injected but every 4-5 days. Make sure you start when your hormonal levels start to crash, not the following day or week after your last inject, unless you are taking something with a small half-life.
I guess you could take it subcut. since so many people do it here and it works for them.
Dosing is usually 1500-3000 iu's no longer than 2-3 weeks. If you were on something light you may need a small initial dose of 1500 iu's, and then 500-1000 every following inject. If doing something heavy androgenic (1g Test), you may want to start with 5000iu's and then follow that up each time with 2500 iu's. Run it with nolvadex or clomid or both.