A manual antenna switch comprises a flip or rotary switch with some antenna sockets. Leads need to be kept short and the whole thing needs to be in an earthed metal case. Connecting wires must be very short (to minimise stray inductance, capacitance and loss) while switch contacts must be heavy enough to withstand sometimes high transmit powers.
You can buy antenna switches commercially. But they can be expensive. But also beware some cheap antenna switches which can be very shoddily built (even if they claim they can do 1 kW!). At least if you build your own you can control what goes into your switch and the quality of workmanship. Things get more critical with higher power levels and higher frequencies. Although note that if using low power a lossy switch won't complain by arcing but it may still be losing valuable watts.
Anyway here's some manual coax switch ideas to get you started. Some amateurs use remote antenna switches but we'll discuss those in a later post.
ZL2IFB's simple instructions for building a manual antenna switch
PA0FRI coax switch
W5CYF's homebrew coax switch (Part 1 of video series - other parts follow)
The above are good if you're using coaxial feeders. If you're using open-wire feedline then a knife switch is what you want.
PS: Want to read more about antennas? Consider this selection of antenna books. They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.