I am installing a new SSD in my late 2011 macbook pro and have updated to latest High Sierra 10.13.6. I would just like to install the new ssd and save the old HD. A fresh start. Can i just simply remove old HD and format and install new SSD? Thanks

That should be fine. You will need to boot into Recovery Mode (Command + R) or Internet Recovery Mode (Command + Option + R) to boot into the High Sierra installer. Just make sure to erase the physical SSD as GUID partition and APFS (the first option). If the SSD is not shown in the left pane of Disk Utility, then click on "View" and select "Show all devices".

You can also create a bootable macOS USB installer using these instructions:

https://support.apple.com/en-us/HT201372

Should be fine. There are a bunch of ways you can do it, either with the recovery partition (if you've got it) or Internet Recovery.

You could get an external USB enclosure/caddy for the SSD and format it in APFS, then install High Sierra. The advantage to this way is that you could use the recovery partition on your existing drive, and also test the MacOS installation by doing an option boot by holding option during power on and selecting the external drive as the boot volume.

With an enclosure you could also do that installation then boot externally. I think one can option boot an external recovery partition.