How to send a buffer with spi
You'll need to select the slave by asserting SlaveSelect (the 'lat' bit?), then loop, sending the buffer 8 bits at a time, by writing to SPDR and waiting for completion for each byte. Then de-assert SS and you're done. There's no more efficient way than that.

Most likely, SS will be asserted by setting it LOW.

More powerful microcontrollers (I assume you're using an 8-bit AVR?) will be able to send more than 8 bits at a time. But the AVR is limited to 8-bit transfers.