I joined frames for several years with Stanley miter vises before finally buying underpinners. In my limited experience, almost any method of gluing & nailing may be acceptable if it is done right. That said, I'm not sure about the tensioned string mentioned by one of the above. The main difference is convenience and labor time, but an underpinner is definitely the best way to go, as soon as you can afford it.
I preferred to use four vises. First, I would fit up all four corners to make sure there were no unnoticed warps or flawed cuts. Some chop services are better than others, so you might want to invest in a miter sanderif you haven't already. That tool will be useful forever.
If all is well withy the fits, then I would loosen two opposing rails and apply glue to both ends of them, leaving the other two rails tight in the vises. With the glued rails carefully positioned and retightened in the vices, the next step is to wipe off oozed glue. A dry toothbrush is OK for this, but a wet one is better, followed by a paper towel.
Always pre-drill. I suggest a real drill bit, which removes material. A brad may be used, too, which simply displaces material, but still reduces ther possibility of splitting the grain, and makes nailing easier.
Whether to cross-nail is a decision to be made based on hardness of wood, area of glued profile, and size of frame. Generally, I like to nail only the top & bottom rails to avoid nail holes showing on the sides. But a big, snotwood, small profile frame would get cross-nailed.
When it's time to upgrade your tools, I suggest spending about $350 for a good pneumatic pinner. (Plus $some for a compressor if you don't have one now.) Mine tool is an Omer, from AMS, which will drive 28-gauge pins up to 1-1/8" long. This sort of tool is much faster than drilling/nailing, and if you get small-diameter pins, the holes are almost invisible. And later, after you get your underpinner, the pinner will still be useful for joining deep shadowbox frames and perhaps for repairing loose miters on frames customers drag in.
And as soon as possible, start saving for purchase of an underpinner. I skipped the manual devices and went straight to a pneumatic machine. The price difference, in my case, was paid back by time savings in about a month. What a deal.