First assuming that compatible hardware is available, get compatible SODIMM RAM. and identify type of hard disk your laptop can use.

General idea, check the laptop user manual to identify what are the maximum RAM that are supported total, what are the maximum RAM supported in a slot. And importantly does the motherboard chipset of your laptop supports SODIMM that are different capacity. Clock speed as long as its higher than the laptop spec, the motherboard chipset will automatically tune the RAM Clock speed to match the maximum speed supported by the motherboard.

For context, the old laptop for upgrade is an Acer Aspire E5-475 which comes with 4GB RAM, 1TB HDD that has an Intel i5-7200U CPU at 2.5GHz.