Installing from tape is probably the easiest method, short of an on-line install using FTP or a CDROM install. The installation program expects the files to be simply tar'ed onto the tape, so after getting all of the files for distribution you are interested in, simply tar them onto the tape with a command like:
cd /freebsd/distdir
tar cvf /dev/rwt0 (or /dev/rst0) dist1 .. dist2
When you go to do the installation, you should also make sure that you leave enough room in some temporary directory (which you will be allowed to choose) to accommodate the full contents of the tape you have created. Due to the non-random access nature of tapes, this method of installation requires quite a bit of temporary storage. You should expect to require as much temporary storage as you have stuff written on tape.
Note: When going to do the installation, the tape must be in the drive before booting from the boot floppy. The installation probe may otherwise fail to find it.