The rope Idea is good to get your primlinary length. Keep in mind you need about 1 3/4" sag in the top of the track between the tires to minimize excessive chain wear.
Here is the 1 man installation procedure that Loggering puts out in video, if you buy a new set from them:
Roll the trac out in straight line
Turn the trac over so gorund contact side of the track is facing up
Drive your machine up one end of the track, get it inline with the track
Connect a chain to first track slat closest to front tire
Run the chain over front wheel and rear wheel and pile rest of chain behind rear wheel
Back the machine straight back until the far end of the track goes over the front tire and falls close to mid-piont between both wheels
Manually lift the rest of the track over the front tire.
Use you strap, connect 1 - 2 slats back from the end over the rear wheel, feed the loose end of the strap under the both end slats and hook on to the front half of track 1 or 2 slats back from the end. This will pull the track together and hold the ends up so bolting them together is easier.